3GPP 24.501 v20.0.0 — the document's own text
8.3.1.7 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).
The UE shall include this IE if the UE requests to establish a PDU session as an always-on PDU session.