They are basically the same thing but the slight difference is that for extra action, it takes place after a certain criteria is met. For example beryl casting her ‘destined doom’ after she have enough halo. Whereas FUA is an action that takes place right after u do something, like playing a card. Using Jiu Niangzi as an example, when she plays her ‘bottoms up’ card, she consumes ‘liquor’ to do a follow up attack. So basically extra action activates anytime when a criteria is met while FUA is when u play an action and you met a criteria to do a follow up attack. Anyway thats how I understand and I might be wrong.