Ulthuan

For the Swordmasters it says "Battleline if army has Eldritch council allegiance". Does that mean that Swordmasters can only be Battleline if all my units are from the Eldritch Council?
But in the end I guess it doesn't matter that much if I do not use Warscroll Battalions.

I think that's how it works. I only had a brief look at the rules, but I think I saw something in there about how you can choose your army's allegiance if there are multiple choices, but you can only choose an allegiance if all your units have that allegiance. So having Order allegiance is fairly easy, while to have Eldritch Council allegiance, all your units have to have that keyword. (Technically speaking, an all-Eldritch Council army could choose to have the Order allegiance, but then it'd be invalid because it doesn't have the minimum battleline units.)

Which is a bit of a pain for most of the aelf microfactions, since sticking entirely within one of those microfactions will lead to an army that's so one-dimensional it's not funny, and as soon as you go outside that tiny list you're back to the generic Order battleline units that are basically core tax.

I guess you could do a house rule that says that if you general is from a certain allegiance than the units from it counts as battleline. But since I have yet to play the game it's a bit early for that.