diff --git a/src/Authorization/AuthorizationDataProviderInterface.php b/src/Authorization/AuthorizationDataProviderInterface.php
index 43a4f5adb8b42591a516ad02f6c3b1578e9b3d98..f0d3b3ff65405e9c466c509fa691f0428cdb532e 100644
--- a/src/Authorization/AuthorizationDataProviderInterface.php
+++ b/src/Authorization/AuthorizationDataProviderInterface.php
@@ -7,9 +7,20 @@ namespace Dbp\Relay\CoreBundle\Authorization;
 interface AuthorizationDataProviderInterface
 {
     /**
+     * Returns a list of available attributes.
+     *
      * @return string[]
      */
     public function getAvailableAttributes(): array;
 
-    public function getUserAttributes(string $userIdentifier): array;
+    /**
+     * Returns an array of attribute names and values.
+     * In case an attribute has no value then it isn't contained.
+     *
+     * In case $userIdentifier is null then there is no active user and
+     * only globally available attributes shold be returned.
+     *
+     * @return array<string, mixed>
+     */
+    public function getUserAttributes(?string $userIdentifier): array;
 }