3GPP 24.501 v20.0.0 — the document's own text
8.2.6.8 Uplink data status
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE shall be included if the UE has uplink user data pending to be sent or the UE has active multicast MBS session(s), unless the UE performs a periodic registration updating procedure.