“The different maps inside RA2 maps (which are actually called ‘arenas’; one RA2 map will have multiple arenas therein) are built for different purposes.
Most arenas are designed for 1v1 or 2v2 play, but these days, pickup is most popular (to my knowledge) because there’s not so much waiting.
In an arena designed for, let’s say 2v2, if you connect with 5 other players, the 6 players (yourself + the other five) have to split into 3 teams of 2 players each, and then enter a 2v2 arena. So you have three teams, let’s call them A, B, and C. Team A and Team B will play each other for one round; Team C will spec. Team A wins the round, Team B moves to the back of the line, and Team C enters the arena for the next round and fights Team A. This leads to periods where players/teams are actually not playing. Typically, there’s more playing and less waiting around in pickup arenas (which is what we usually play).
Pickup arenas are basically for Team DM. Everyone who joins can connect, join a pickup arena, and get right to playing from the next round. But there’s usually only one pickup arena per map (some maps have two), and the pickup arenas vary in size from map to map. I’m not too familiar with all the RA2 maps, but there are some maps where you can play 7v7 fairly comfortably, and others where 4v4 feels crowded.”
-angry, in this AcmeCTF forum thread, explaining what makes ra2 maps different in Quake 2: http://www.acmectf.com/forum/index.php/topic,4490.0.html