3GPP 23.501 v20.2.0 — the document's own text
5.9.9 AMF UE NGAP ID
Taught in 7. Every name the system uses (The 5G system architecture, in depth), 4. Knowing who you are and where you are (The 5G system architecture, overview).
An AMF UE NGAP ID is an identifier used to identify the UE in AMF on N2 reference point. AMF allocates the AMF UE NGAP ID and send it to the 5G-AN. For the following N2 signalling interaction sent from 5G-AN to AMF, AMF UE NGAP ID is used to identify the UE at the AMF. AMF UE NGAP ID is unique per AMF set. AMF UE NGAP ID may be updated without AMF change, or with AMF change as specified at clause 5.21.2.2.