Honestly, the international players rarely play that I don't even count the likes of Michael Clarke as part of the NSW squad. But when a usual squad player gets called up you can call in anyone to replace them - from players with a contract with your state to players playing grade cricket (in your state). Not sure what the exact ruling to the letter is, but pretty sure that's it.
After being convinced this version is IMPOSSIBLE in aus domestic with the worst season I have ever had in any ICC game (last OD, 2nd last FC - didn't win a FC game (3 losses 7 draws) and only winning two OD's) my second season has been a huge improvement. First in OD after 7 games (6 wins 1 loss) but FC is still ████, 6 games 6 draws! I can't win on FC. The pitches are too good, every innings is 300+ almost.