The Flagship lounges would be better called International First Class lounge are in certain airports are for International flights only - and require either a 1st class ticket (no business class tixs), or you have to be a AA EXP and can enter from any class or on One World top tier passenger - and can enter from from any class ticket.

I'm no longer certain about AC entry - used to be ticket catagory (first class) did not get you entry.

Finally you can sometimes buy a one day AC pass at the lounge. Hope this helps.

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Askworldtraveler, EXP,4 million AA miles+++