3GPP 24.501 v20.0.0 — the document's own text

8.3.7.5 Always-on PDU session requested

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 in the message for a PDN connection established when in S1 mode, after an inter-system change from S1 mode to N1 mode, if the UE is a UE operating in single-registration mode in a network supporting N26 interface, the UE has not previously successfully performed the UE-requested PDU session modification to provide this capability, and the UE requests the PDU session to be an always-on PDU session in the 5GS.