3GPP 24.501 v20.0.0 — the document's own text
8.2.6.40 Unavailability information
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 an event is triggered in the UE that would make the UE unavailable for a certain period.