diff --git a/src/classes/MDNodaLink.php b/src/classes/MDNodaLink.php
new file mode 100644
index 0000000..fd0f72e
--- /dev/null
+++ b/src/classes/MDNodaLink.php
@@ -0,0 +1,34 @@
+<?PHP
+/**
+ * Describes a reference to an external norm data repository.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Describes a reference to an external norm data repository.
+ */
+final class MDNodaLink {
+
+    public readonly MDNodaRepository $source;
+    public readonly string $id;
+
+    /**
+     * Constructor.
+     *
+     * @param MDNodaRepository $source Source repository.
+     * @param string           $id     ID.
+     *
+     * @return void
+     */
+    public function __construct(MDNodaRepository $source, string $id) {
+
+        $this->source = $source;
+        if (($validatedId = $this->source->validateId(trim($id))) === false) {
+            throw new MDInvalidNodaLink("Invalid noda ID / link: " . $id . " [Repository: " . $this->source->toDbName() . "]");
+        }
+        $this->id = $validatedId;
+
+    }
+}
diff --git a/src/classes/README.md b/src/classes/README.md
new file mode 100644
index 0000000..c2e68d4
--- /dev/null
+++ b/src/classes/README.md
@@ -0,0 +1 @@
+# Full-scale data types that provide further functionality but end up representing base data types
diff --git a/src/enums/MDNodaRepository.php b/src/enums/MDNodaRepository.php
new file mode 100644
index 0000000..9e15ca6
--- /dev/null
+++ b/src/enums/MDNodaRepository.php
@@ -0,0 +1,426 @@
+<?PHP
+/**
+ * Represents an external norm data repository.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Represents an external norm data repository.
+ */
+enum MDNodaRepository implements MDValueEnumInterface, JsonSerializable {
+
+    case allgemein; // General link to a good source
+    case aat;
+    case ackerbau;
+    case bne;
+    case bnf;
+    case cona;
+    case editionhumboldtdigital;
+    case gnd;
+    case grobsystematik;
+    case iconclass;
+    case lcsh;
+    case loc; // Duplicate?
+    case mbl;
+    case mindatorg;
+    case moebeltypologie;
+    case ndb_adb;
+    case ndl;
+    case ndp_ikmk;
+    case nomisma;
+    case npg;
+    case oberbegriffsdatei;
+    case orcid;
+    case osm;
+    case pim;
+    case pleiades;
+    case rkd;
+    case ulan;
+    case viaf;
+    case wikidata;
+    case wikipedia;
+
+    /**
+     * Returns a value of this type based on a string.
+     *
+     * @param string $input Input to get a value from.
+     *
+     * @return MDNodaRepository
+     */
+    public static function fromString(string $input):MDNodaRepository {
+
+        return match($input) {
+            'allgemein' => self::allgemein, // General link to a good source
+            'aat'  => self::aat,
+            'ackerbau'  => self::ackerbau,
+            'bne'       => self::bne,
+            'bnf'       => self::bnf,
+            'cona'      => self::cona,
+            'edition humboldt digital' => self::editionhumboldtdigital,
+            'gnd'       => self::gnd,
+            'grobsystematik' => self::grobsystematik,
+            'iconclass' => self::iconclass,
+            'lcsh'      => self::lcsh,
+            'loc'       => self::loc, // Duplicate?
+            'mbl'       => self::mbl,
+            'MBL'       => self::mbl,
+            'mindat.org' => self::mindatorg,
+            'moebeltypologie' => self::moebeltypologie,
+            'NDB/ADB'   => self::ndb_adb,
+            'ndl'       => self::ndl,
+            'ndp-ikmk'  => self::ndp_ikmk,
+            'nomisma'   => self::nomisma,
+            'npg'       => self::npg,
+            'oberbegriffsdatei' => self::oberbegriffsdatei,
+            'orcid'     => self::orcid,
+            'osm'       => self::osm,
+            'pim'       => self::pim,
+            'pleiades'  => self::pleiades,
+            'rkd'       => self::rkd,
+            'ulan'      => self::ulan,
+            'viaf'      => self::viaf,
+            'wikidata'  => self::wikidata,
+            'Wikidata'  => self::wikipedia,
+            default => throw new MDpageParameterNotFromListException("Unknown noda repository: " . $input),
+        };
+
+    }
+
+    /**
+     * Returns the name as stored in the DB.
+     *
+     * @return string
+     */
+    public function toDbName():string {
+
+        return match($this) {
+            self::allgemein => 'allgemein', // General link to a good source
+            self::aat => 'aat',
+            self::ackerbau => 'ackerbau',
+            self::bne => 'bne',
+            self::bnf => 'bnf',
+            self::cona => 'cona',
+            self::editionhumboldtdigital => 'edition humboldt digital',
+            self::gnd => 'gnd',
+            self::grobsystematik => 'grobsystematik',
+            self::iconclass => 'iconclass',
+            self::lcsh => 'lcsh',
+            self::loc => 'loc',
+            self::mbl => 'MBL',
+            self::mindatorg => 'mindat.org',
+            self::moebeltypologie => 'moebeltypologie',
+            self::ndb_adb => 'NDB/ADB',
+            self::ndl => 'ndl',
+            self::ndp_ikmk => 'ndp-ikmk',
+            self::nomisma => 'nomisma',
+            self::npg => 'npg',
+            self::oberbegriffsdatei => 'oberbegriffsdatei',
+            self::orcid => 'orcid',
+            self::osm => 'osm',
+            self::pim => 'pim',
+            self::pleiades => 'pleiades',
+            self::rkd => 'rkd',
+            self::ulan => 'ulan',
+            self::viaf => 'viaf',
+            self::wikidata => 'wikidata',
+            self::wikipedia => 'Wikidata',
+        };
+
+    }
+
+    /**
+     * Returns the base of the repository's entity page URLs.
+     *
+     * @param string $id Identifier to link to.
+     *
+     * @return string
+     */
+    public function getEntityLink(string $id):string {
+
+        return match($this) {
+            self::allgemein => $id,
+            self::aat       => 'https://vocab.getty.edu/page/aat/' . $id,
+            self::ackerbau  => 'https://term.museum-digital.de/ackerbau/tag/' . $id,
+            self::bne       => 'http://datos.bne.es/persona/' . $id,
+            self::bnf       => "https://catalogue.bnf.fr/ark:/12148/cb" . $id,
+            self::cona      => 'https://vocab.getty.edu/page/cona/' . $id,
+            self::editionhumboldtdigital => 'https://edition-humboldt.de/register/personen/detail.xql?normid=http://d-nb.info/gnd/' . $id,
+            self::gnd       => 'https://d-nb.info/gnd/' . $id,
+            self::grobsystematik => 'https://term.museum-digital.de/grobsystematik/tag/' . $id,
+            self::iconclass => 'http://iconclass.org/rkd/' . $id,
+            self::lcsh      => 'http://id.loc.gov/authorities/names/' . $id,
+            self::loc       => 'http://id.loc.gov/authorities/names/' . $id,
+            self::mbl       => 'http://www.uni-magdeburg.de/mbl/PHP_Skripte/mbl_verwaltung/mbl_verw_anzeige_biog.php?auswahl=3&liste_biog_name=' . $id,
+            self::mindatorg => 'https://www.mindat.org/min-' . $id . '.html',
+            self::moebeltypologie => 'https://term.museum-digital.de/moebel/tag/' . $id,
+            self::ndb_adb   => 'https://www.deutsche-biographie.de/pnd' . $id . '.html',
+            self::ndl       => 'https://id.ndl.go.jp/auth/ndlna/' . $id,
+            self::ndp_ikmk  => 'https://ikmk.smb.museum/ndp/land/' . $id,
+            self::nomisma   => 'http://nomisma.org/id/' . $id,
+            self::npg       => 'https://www.npg.org.uk/collections/search/person/' . $id,
+            self::oberbegriffsdatei => 'https://term.museum-digital.de/oberbegriffsdatei/tag/' . $id,
+            self::orcid     => 'https://orcid.org/' . $id,
+            self::osm       => 'https://www.openstreetmap.org/relation/' . $id,
+            self::pim       => 'https://opac-nevter.pim.hu/en/record/-/record/' . $id,
+            self::pleiades  => 'https://pleiades.stoa.org/places/' . $id,
+            self::rkd       => 'https://rkd.nl/explore/artists/' . $id,
+            self::ulan      => 'https://vocab.getty.edu/page/ulan/' . $id,
+            self::viaf      => 'https://viaf.org/viaf/' . $id,
+            self::wikidata  => 'https://www.wikidata.org/wiki/' . $id,
+            self::wikipedia => 'https://de.wikipedia.org/wiki/' . $id,
+        };
+
+    }
+
+    /**
+     * Validates a numeric ID, returning a string or false.
+     *
+     * @param string   $id       ID to validate.
+     * @param string[] $prefixes Prefixes to strip off.
+     *
+     * @return string|false
+     */
+    private static function validateNumericId(string $id, array $prefixes):string|false {
+
+        if (filter_var($id, FILTER_VALIDATE_URL) !== false) {
+            $toRemove = [];
+            foreach ($prefixes as $prefix) $toRemove[$prefix] = "";
+            $id = strtr($id, $toRemove);
+        }
+
+        if (filter_var($id, FILTER_VALIDATE_INT) === false) {
+            return false;
+        }
+
+        return $id;
+
+    }
+
+    /**
+     * Validates a iconclass ID, returning a string or false.
+     *
+     * @param string $id ID to validate.
+     *
+     * @return string|false
+     */
+    private static function validateIconclassId(string $id):string|false {
+
+        if (filter_var($id, FILTER_VALIDATE_URL) !== false) {
+            $id = strtr($id, ['http://iconclass.org/rkd/' => '', 'http://iconclass.org/' => '', 'https://iconclass.org/' => '']);
+        }
+
+        if (preg_match("/^[0-9a-z\/]*$/", $id) === false) return false;
+
+        return $id;
+
+    }
+
+    /**
+     * Validates a GND ID, returning a string or false.
+     *
+     * @param string   $id       ID to validate.
+     * @param string[] $prefixes Prefixes to strip off.
+     *
+     * @return string|false
+     */
+    private static function validateGndId(string $id, array $prefixes):string|false {
+
+        if (filter_var($id, FILTER_VALIDATE_URL) !== false) {
+            $toRemove = [];
+            foreach ($prefixes as $prefix) $toRemove[$prefix] = "";
+            $id = strtr($id, $toRemove);
+        }
+
+        if (preg_match("/^[0-9-X]*$/", $id) === false) return false;
+
+        return $id;
+
+    }
+
+    /**
+     * Validates a Library of Congress name ID, returning a string or false.
+     *
+     * @param string $id ID to validate.
+     *
+     * @return string|false
+     */
+    private static function validateLocId(string $id):string|false {
+
+        if (filter_var($id, FILTER_VALIDATE_URL) !== false) {
+            $id = strtr($id, ['http://id.loc.gov/authorities/names/' => '']);
+        }
+
+        if (substr($id, 0, 1) !== 'n') {
+            throw new MDgenericInvalidInputsException("LOC IDs must start with n");
+        }
+
+        if (filter_var(substr($id, 1), FILTER_VALIDATE_INT) === false) {
+            return false;
+        }
+
+        return $id;
+
+    }
+
+    /**
+     * Validates a LCSH ID, returning a string or false.
+     *
+     * @param string $id ID to validate.
+     *
+     * @return string|false
+     */
+    private static function validateLcshId(string $id):string|false {
+
+        if (filter_var($id, FILTER_VALIDATE_URL) !== false) {
+            $id = strtr($id, ['http://id.loc.gov/authorities/names/' => '']);
+        }
+
+        if (substr($id, 0, 2) !== 'sh') {
+            throw new MDgenericInvalidInputsException("LCSH IDs must start with sh");
+        }
+
+        if (filter_var(ltrim(substr($id, 2), "0"), FILTER_VALIDATE_INT) === false) {
+            return false;
+        }
+
+        return $id;
+
+    }
+
+    /**
+     * Validates a wikidata ID, returning a string or false.
+     *
+     * @param string $id ID to validate.
+     *
+     * @return string|false
+     */
+    private static function validateWikidataId(string $id):string|false {
+
+        if (filter_var($id, FILTER_VALIDATE_URL) !== false) {
+            $id = strtr($id, ['https://www.wikidata.org/wiki/' => '']);
+        }
+
+        if (substr($id, 0, 1) !== 'Q') {
+            throw new MDgenericInvalidInputsException("Wikidata IDs must be Q IDs - and start with that letter");
+        }
+
+        if (filter_var(substr($id, 1), FILTER_VALIDATE_INT) === false) {
+            return false;
+        }
+
+        return $id;
+
+    }
+
+    /**
+     * Validates an ID.
+     *
+     * @param string $id Identifier to link to.
+     *
+     * @return string|false
+     */
+    public function validateId(string $id):string|false {
+
+        return match($this) {
+            self::allgemein => filter_var($id, FILTER_VALIDATE_URL),
+            self::aat       => self::validateNumericId($id, ['https://vocab.getty.edu/page/aat/']),
+            self::ackerbau  => self::validateNumericId($id, ['https://term.museum-digital.de/ackerbau/tag/']),
+            self::bne       => self::validateNumericId($id, ['http://datos.bne.es/persona/']),
+            self::bnf       => self::validateNumericId($id, ["https://catalogue.bnf.fr/ark:/12148/cb"]),
+            self::cona      => self::validateNumericId($id, ['https://vocab.getty.edu/page/cona/']),
+            self::editionhumboldtdigital => self::validateGndId($id, ['https://edition-humboldt.de/register/personen/detail.xql?normid=http://d-nb.info/gnd/']),
+            self::gnd       => self::validateGndId($id, ['https://d-nb.info/gnd/']),
+            self::grobsystematik => self::validateNumericId($id, ['https://term.museum-digital.de/grobsystematik/tag/']),
+            self::iconclass => self::validateIconclassId($id),
+            self::lcsh      => self::validateLcshId($id),
+            self::loc       => self::validateLocId($id),
+            self::mbl       => self::validateNumericId($id, ['http://www.uni-magdeburg.de/mbl/PHP_Skripte/mbl_verwaltung/mbl_verw_anzeige_biog.php?auswahl=3&liste_biog_name=']),
+            self::mindatorg => self::validateNumericId($id, ['https://www.mindat.org/min-', '.html']),
+            self::moebeltypologie => self::validateNumericId($id, ['https://term.museum-digital.de/moebel/tag/']),
+            self::ndb_adb   => self::validateGndId($id, ['https://www.deutsche-biographie.de/pnd', '.html']),
+            self::ndl       => self::validateNumericId($id, ['https://id.ndl.go.jp/auth/ndlna/']),
+            self::ndp_ikmk  => self::validateNumericId($id, ['https://ikmk.smb.museum/ndp/land/']),
+            self::nomisma   => self::validateNumericId($id, ['http://nomisma.org/id/']),
+            self::npg       => self::validateNumericId($id, ['https://www.npg.org.uk/collections/search/person/']),
+            self::oberbegriffsdatei => self::validateNumericId($id, ['https://term.museum-digital.de/oberbegriffsdatei/tag/']),
+            self::orcid     => preg_match('/^[0-9]{4}-[0-9]{4}-[0-9]{4}-[0-9]{4}$/', $id) ? $id : false,
+            self::osm       => self::validateNumericId($id, ['https://www.openstreetmap.org/relation/']),
+            self::pim       => self::validateNumericId($id, ['https://opac-nevter.pim.hu/en/record/-/record/']),
+            self::pleiades  => self::validateNumericId($id, ['https://pleiades.stoa.org/places/']),
+            self::rkd       => self::validateNumericId($id, ['http://rkd.nl/explore/artists/', 'https://rkd.nl/explore/artists/']),
+            self::ulan      => self::validateNumericId($id, ['http://vocab.getty.edu/ulan/', 'http://vocab.getty.edu/page/ulan/', 'https://vocab.getty.edu/page/ulan/']),
+            self::viaf      => self::validateNumericId($id, ['https://viaf.org/viaf/']),
+            self::wikidata  => self::validateWikidataId($id),
+            self::wikipedia => str_replace('https://de.wikipedia.org/wiki/', '', $id),
+        };
+
+    }
+
+    /**
+     * Lists all available names.
+     *
+     * @return array<string>
+     */
+    public static function caseNames():array {
+
+        $output = [];
+
+        $cases = self::cases();
+        foreach ($cases as $case) {
+            $output[] = $case->name;
+        }
+
+        return $output;
+
+    }
+
+    /**
+     * Gets an unsorted list of the entries in a translated version.
+     *
+     * @param MDTlLoader $tlLoader Translation loader.
+     *
+     * @return array<string, string>
+     */
+    public static function getUnsortedList(MDTlLoader $tlLoader):array {
+        return MDValueSet::getTlUnsortedList($tlLoader, self::caseNames(), "attendance_status_set", "attendance_status_set");
+
+    }
+
+    /**
+     * Gets a sorted list of the entries in a translated version.
+     *
+     * @param MDTlLoader $tlLoader Translation loader.
+     *
+     * @return array<string, string>
+     */
+    public static function getSortedList(MDTlLoader $tlLoader):array {
+        return MDValueSet::getTlSortedList($tlLoader, self::caseNames(), "attendance_status_set", "attendance_status_set");
+
+    }
+
+    /**
+     * Returns the name of the current value in translation.
+     *
+     * @param MDTlLoader $tlLoader Translation loader.
+     *
+     * @return string
+     */
+    public function getTledName(MDTlLoader $tlLoader):string {
+
+        return $tlLoader->tl("attendance_status_set", "attendance_status_set", $this->name);
+
+    }
+
+    /**
+     * Provides the option to serialize as a string during json_encode().
+     *
+     * @return string
+     */
+    public function jsonSerialize():string {
+
+        return $this->name;
+
+    }
+}