3GPP 24.501 v20.0.0 — the document's own text
8.3.7.2 5GSM capability
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE is included in the message when UE needs to indicate the 5GSM capability IE as defined in clause 6.4.2.2.