NSSF
Network Slice Selection Function: the 5GC function that selects the appropriate network slice instance and AMF set for a UE based on the requested S-NSSAI.
Slice selection isn't always obvious, and that's the gap the NSSF fills. When a UE registers with a set of requested S-NSSAIs, the AMF may not be able to resolve which slice instances should serve it — especially when slices are restricted by location, by roaming agreement, or when the current AMF set doesn't cover the requested slice. It asks the NSSF.
The NSSF works out the allowed NSSAI for that UE in that registration area, maps S-NSSAIs to the right Network Slice Instances, and tells the AMF which AMF set should be serving — which can trigger an AMF reselection. You won't see it on every call flow; many deployments run a small number of broad slices where the AMF resolves selection locally and never consults it. It earns its keep once you have many slices with real geographic or subscription constraints.
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