DCA construction has altered some arrival and departure operations for American Airlines. American Airlines used to fly exclusively out of Terminal C and is now shifting some operations to B/C known as “Central Pier.”

https://www.ifly.com/ronald-reagan-washington-national-airport#termGuide

Terminal B and C

Terminal B and C are the newest and the largest terminals of the airport. Both terminals have the same structure and have three concourses.

Terminal B has gates numbered from 10-22, and the airlines operated here are Alaska Airlines, Delta, and United.

Terminal B/C, gates 23-34, houses American and JetBlue Airlines.

Terminal C, gates 35-45, is exclusive to American Airlines.