Originally Posted by
christep
Your US domestic flights should be in A if there's A availability on the flight. ...
... just to mention the proviso that this is true
only if there is no J cabin on the aircraft.
Most AA domestic flights have the two cabin classes, First class and Coach. It's for these flight you book into A (First).
Some (Mainly 'trascontinental') flights have three cabin classes, First class, Business class and Coach. For these you book into D (Business).