diff --git a/sru/onbsru.py b/sru/onbsru.py
index f9f34befeda193ac35ffec659376e32843b442eb..335b38bdbfe43366ee90f56f413d2c59fb9d7325 100644
--- a/sru/onbsru.py
+++ b/sru/onbsru.py
@@ -14,12 +14,17 @@ class OnbRecordRetriever(almasru.RecordRetriever):
                  xml_format: str = 'marcxml'):
         super().__init__(subdomain, inst_code, xml_format)
 
+    # Create alias for querying AC-numbers
     def by_ac_num(self, ac_num: str):
         return self.by_marc_009(ac_num)
 
-    def get_pids_for_ac(self, ac_num: str):
+    def get_pids_for_ac(self, ac_num: str) -> list:
         """
         Extract all PIDs from XML based on MARC 856 4 2.
+        PIDs are identifiers migrated from a legacy system.
+
+        :param ac_num: ID used in Austrian Library Network
+        :return: List of identifiers.
         """
 
         namespaces = {