Not 100% familiar with AA
, however some airlines like Continental and United have contract that lump together several types of aircraft for pay purposes.
, 737-300/500 is one type, 737NG/MD-80/B757-200 is another, while the 757-300/767-200/400/777 is the third.
, the 737/A320 is one, the 757/767 is another, while the 777 and 747-400 are the third.
By lumping together and reducing types of equipment for pay purposes the desire to keep swapping to a larger and larger aircraft in ones career is reduced. For instace at both CO
, you would really only have two upwards steps to take versus and airline that pays differently for each aircraft they own.
This ultimately saves the airline million of dollars in reduced training cycles.