TYPO3 Core

In the context of using Configuration/Extbase/Persistence/Classes.php, Extbase allows to extend a domain model, e.g.:

namespace VENDOR\MyExt\Domain\Model;

class User extends FrontendUser
{

    /**
     * @var string
     */
    protected $courseStudy;

    // getter/setter

Corresponding definition looks like:

return [
    \TYPO3\CMS\Extbase\Domain\Model\FrontendUser::class => [
        'tableName' => 'fe_users',
        'recordType' => '\TYPO3\CMS\Extbase\Domain\Model\FrontendUser',
        'subclasses' => [
            'Tx_MyExt_User' => \VENDOR\MyExt\Domain\Model\User::class,
        ]
    ],
    \VENDOR\MyExt\Domain\Model\User::class => [
        'tableName' => 'fe_users',
        'properties' => [
            'courseStudy' => [
                'fieldName' => 'tx_myext_course_study'
            ],
        ],
    ],
];

This way the mapping just works fine!

However if you strong type the property, then it doesn't work anymore:

namespace VENDOR\MyExt\Domain\Model;

class User extends FrontendUser
{

    /**
     * @var string
     */
    protected string $courseStudy;

    // getter/setter

(mind the protected string).