What I've heard on flights with USA-based carriers (including AA) is that the item must fit totally under the seat in front of you. Ie, I've had them tell me to either make straps from a bag go under the seat or put the bag overhead, several years ago (when I had a bag, which I no longer use, that had "too many" extra straps).

Meanwhile, add LAN to the list of foreign airlines which don't allow anything at all under the seat in exit rows during takeoff/landing.