I really don't understand AA's staffing on the A321. Sometimes one of the three Y FAs go up to F to assist during the meal service, but most of the time it's just the one FA handling all 20 passengers.
They staff four FAs on the A321s. Each FA is assigned a number (this determines where they are working). #1 is the lead FA (FC), and #3 is the FA who does the pre-departure drinks. #3 is to help the FC FA, with partial meal service, then go to Y and help them. If AA would staff, 5 FA's on the A321's service would be better but they won't do it.