3GPP 24.501 v20.0.0 — the document's own text
8.3.9.5 Always-on PDU session indication
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The network shall include this IE if the network decides to inform the UE whether the PDU session is an always-on PDU session.