diff --git a/basyx/aas/model/base.py b/basyx/aas/model/base.py
index e96bf4a1d..a39f48fe9 100644
--- a/basyx/aas/model/base.py
+++ b/basyx/aas/model/base.py
@@ -2130,6 +2130,9 @@ class SpecificAssetId(HasSemantics):
     A specific asset ID describes a generic supplementary identifying attribute of the asset.
     The specific asset ID is not necessarily globally unique.
 
+    *Constraint AASd-133:* SpecificAssetId/externalSubjectId shall be a global reference,
+        i.e. Reference/type = ExternalReference
+
     :ivar name: Key of the identifier
     :ivar value: The value of the identifier with the corresponding key.
     :ivar external_subject_id: The (external) subject the key belongs to or has meaning to.