EMP shielding. Nuclear explosions release an absolutely insane amount of electromagnetic energy and it fries electronics. This is actually where the first EMP was observed - a high altitude nuclear test fried a bunch of electronics. The idea is that it needs to be able to keep flying even if a nuclear blast goes off even quite some distance away. There's obviously no way to actually nuke proof a plane, it would get vaporized or knocked out of the sky if it were close to the epicenter.

I think they have systems in place to stop EMPs from affecting and potentially taking down AirForce One, and nuclear blasts create EMPs that if left otherwise unshielded would do exactly that.

i gonna make an assumption here and say probably EMP resistance?

That's just the shit they are willing to tell you about.

I legit would not be surprised if there was a secret ejection capsule for the president and some staff that could rocket out of the plane and get caught in mid-air by one of the escort planes.