3GPP 24.501 v20.0.0 — the document's own text
8.2.6.2 Non-current native NAS key set identifier
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 has a valid non-current native 5G NAS security context when the UE performs an inter-system change from S1 mode to N1 mode in 5GMM-CONNECTED mode and the UE uses a mapped 5G NAS security context to protect the REGISTRATION REQUEST message.