3GPP 24.501 v20.0.0 — the document's own text
8.2.6.36 NID
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 5G-GUTI in the 5GS mobile identity IE was assigned by an SNPN other than the SNPN with which the UE is registering.