The game just told me about EX attacks, and I've noticed as I execute attacks two of my characters (nepgear and IF I believe) have an EX> symbol on one attack option, indicating it's the next part of an EX combo. Do I "learn" which combos are EX combos to make this symbol appear? If so, how do I learn them? If not, why does that symbol appear?
As your characters levels, you will unlock EX Finishers. Hitting the correct combos unleashes a special bonus attack at the end of your attack string.
The combo you're doing needs to accure a certain amount of AP in order to be used. The initial attack (the 12 AP X Attack) doesn't count towards this requirement.
You can tell if you have enough AP to use a EX Finisher once you start your combo, as there will be an "EX" tag next to the inputs that guides you to your EX Finisher, letting you know you will have enough AP.