I was recently looking for a field I could use in PFCP to denote the VRF / Network Segment to be used, and initially thought Network Instance would be perfect for this.

It’s not.

Network Instance is kinda preferred over the APN/DNN for decisions, for example a Packet Detection Rule (PDR) does not give a damn what you’ve set as the APN/DNN, only what the Network Instance is set to:

a combination of the parameters, that incoming packets are requested to match, among: Local F-TEID, Network
Instance, UE IP address(es), SDF Filter(s) and/or Application ID. For 5GC, the PDI may additionally contain
one or more QFI(s) to detect traffic pertaining to specific QoS flow(s), Ethernet Packet Filter(s) and/or Ethernet
PDU Session Information (see clause 5.13.1).

TS 129 244 – 5.2.1A Packet Detection Rule Handling

So the PDRs actually look at Network Instance not APN/DNN.

Well, back to the drawing board…