I’m probably in the wrong forum but it’s somewhere to start before being redirected. Can you fly military aircraft with a military callsign within Vatsim, specifically a RAF Poseidon P8. I see guys flying F18’s but have no idea how they generate the required Vatsim credentials.
You can fly aircraft typically used my militaries and use callsigns typically used for military flights just fine. What’s restricted is neither the aircraft type nor the callsign (although both have certain exceptions) but the type of operation.
I’d recommend you read the Special Operations Policy, specifically 2.1.7, to learn more about which activities are restricted.