WAS is just an IATA airport code for the three major Washington, DC region airports to make it easier to search, refer to them, etc. instead of separately.

United considers them all three coterminals for Washington, DC MP promotions unless specified.

United is very strict and specific when it comes to coterminal airports that you can switch around and fly in/out of standby at your discretion. You cannot buy BWI-ORD and show up at IAD or DCA and fly standby to ORD. However, in the past, you could buy IAD-ORD and show up and fly standby DCA-ORD, and vice versa. I have done this in the past and would like to find out if anyone has done it recently or knows if that is still the case.