The Team Respawn Queue dictates what classes will spawn in when the game starts.
If 7 players are in the match, the game will get the first 7 classes in the Team Respawn Queue and randomly set one player to each of the first 7 classes. If 19 players are in the match, it will do the same thing, but with the first 19 classes.
The Team Respawn Queue doesn't use the same class ID system as the rest of the game. It is as follows: