Pimcore\Model\Object\ClassDefinition\Data\Relations\AbstractRelations::allowDocumentRelation PHP Method

    protected function allowDocumentRelation($document)
    {
        $allowedDocumentTypes = $this->getDocumentTypes();
        $allowed = true;
        if (!$this->getDocumentsAllowed()) {
            $allowed = false;
        } elseif ($this->getDocumentsAllowed() and is_array($allowedDocumentTypes) and count($allowedDocumentTypes) > 0) {
            //check for allowed asset types
            $allowedTypes = [];
            foreach ($allowedDocumentTypes as $t) {
                if ($t['documentTypes']) {
                    $allowedTypes[] = $t['documentTypes'];
                }
            }
            if (!in_array($document->getType(), $allowedTypes) && count($allowedTypes)) {
                $allowed = false;
            }
        } else {
            //don't check if no allowed document types set
        }
        Logger::debug("checked object relation to target document [" . $document->getId() . "] in field [" . $this->getName() . "], allowed:" . $allowed);
        return $allowed;
    }