Originally Posted by
Horace
It's about AA squeezing as many seats on the aircraft as will fit, while dealing with then location of exit rows and lavatories.
See
https://www.aerolopa.com/aa-airbus-a321nx (at
AeroLOPA).
On the A-B-C side, between the wing exit row seats and the rear lavatory, AA only has room for regular economy rows.
On the D-E-F side, where there's no rear lavatory, AA has an additional row of seats -- and the two rows behind the wing exit row seats are MCE.
EDITED to add: Oops! I just saw that
PHL already gave the same answer.
Thanks
PHL and
Horace . It seems AA has two A321neo configurations. The A321NX and the A321NX Kodiak. The Kodiak one uses rows 18 & 19 DEF as MCE while the other one has rows18-21 DEF as MCE. According to AA's website the A321neo has 47 MCE's which (by my count) would include rows 18-21.