Both, I think, but I could be mistaken. I think those are basically the pinnacle unit of the exo wave if you've got a wave that is that huge.
In theory, yes, but in practice it's probably physically impossible to get a mothership to spawn except in an AI-homeworld-defense exo that gets spawned in Fallen Spire if you attack an AI homeworld with a serious force. That spawn's budget is the sum of a percentage of all the offensive strikeforces provoked by spire cities up to that point, and it also focuses it all into a single battlegroup, AND it heavily weights its budget towards the lead ship, which is basically the only way to get _that_ many points funneled into a single ship choice.
If it could occur normally you'd basically be up a creek; when engaging from maximum range it can take out a full spire capital fleet.