Apart from the 'standard' A321neo there are additional variants. The first is the A321neo ACF, in which ACF means Airbus Cabin Flex. On this version the second pair of (large) emergency exits in front of the wing is replaced by one or two smaller emergency exits over the wings, while the third pair of emergency doors can be moved aft. Both allow the cabin be configured more flexibly and with more seats. In the official designation they have an 'X' as suffix, so A321-25*NX or A321-27*NX.
https://www.aircraftrecognitionguide.com/airbus-a320-family