As a true FlyerTalker, I'm assuming that the OP reviewed the appropriate thread on FT prior to booking (and paying extra)?
https://www.flyertalk.com/forum/amer...ce-seat-9.html
The Wikipost specifies this:
Row 12 (MCE). Due to a system limitation, AA (mostly incorrectly) has this coded as an exit row on the seat map. It is not. 12 A, B, C, D, and E are standard MCE seats. 12F has no seat in front of it and technically has a direct path to the exit (hence the legal requirement they designate at least that side as an exit row). The underseat storage under 10F is not easily accessible due to the presence of a FA jumpseat.