3GPP 24.501 v20.0.0 — the document's own text
8.3.1.5 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 the UE requests to establish a new PDU session or to transfer an existing PDN connection and any of the 5GSM capabilities supported by the UE is relevant for the PDU session.