It’s a ratio of 321’s in the total fleet. You don’t have to physically fly the 321 to get 321 override. It’s a percentage/ratio of your total Credit hours.

“The rates of pay for Airbus pilots shall be based on the ratio of total system-wide A321 block time to the sum of system-wide A319/ A320 and A321 block time (revenue and non-revenue) in the month. For example, in September 2005, 2,400 block hours (50 percent) are flown in the A321, and 2,400 combined block hours (50 percent) are flown in the A319/A320. Therefore, all Airbus pilots shall be paid 105 percent of their rate of pay set forth in Section 3 of the Basic Agreement, regardless of which aircraft they are assigned/awarded, for all credit earned in the month. The overage shall be paid on the second paycheck in the following month, in this example, on the second paycheck in October 2005.”