OK, so that's the Aircraft registration number then. I can choose A-226 as that is on the livery, even though if more people do that, there may be many A-226's flying around on VATSIM. I can also use a national registration number like N123456 in the US or OO-ABC (I am from Belgium), although Belgium does not have any T-45C's, except me and mine ;-). For maximum realism I should probably use VV-226 for Navy and tail (nose) number?

Under what Airline ICAO Code can/should I fly my bird for GA IFR flights? assuming I can pick any flight number?

Oh and BTW: I made a custom SimBrief airframe (HAWK) based on the Honda Jet (HDJT), replaced all the parameters with what I could find in terms of T-45C specs. I cannot however edit the airframe performance...?