None.
If you want accuracy, it’s hardly a game. If you want a game, by definition it’s hardly accurate. Would you really enjoy being railroaded into history despite your best effort?
Actually, I'd probably get some perverse enjoyment out of a game written by some extreme determinists (perhaps some form of Marxist historians) that kept forcing history back on course in second order counterfactuals no matter what I did.
That aside, by realism I'm referring to the attempt to stick as close to what actually would have been possible or likely, just like (good) AH is supposed to do. For instance, a wargame where Sealion works isn't realistic -- not just because Sealion never happened in real life, but also because an attack wouldn't have succeeded even if the German military had tried it.