3GPP 24.501 v20.0.0 — the document's own text
8.3.2.5 S-NSSAI
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 when the SMF received from the AMF an S-NSSAI together with the PDU SESSION ESTABLISHMENT REQUEST message, the PDU session is a non-emergency PDU session and the UE is not registered for onboarding services in SNPN.