3GPP 24.501 v20.0.0 — the document's own text
8.3.2.6 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 established as an always-on PDU session.