When you put an in the field AVAILABLE, the program will assume this player shall not participate in the next round, and will exclude him from the pairing. He will instead be assigned a WO at the next pairing.
You may of course set this back to later, if this player returns and is allowed to play a later round.
For team events, this field is used to manage exchanges of stand-by team members. For example, if the board-1 player is defined as not availiable, the rest of the team will promote one board and the first stand-by player will enter the lowest board (if you don't change the order).
For individual round-robin events this field has no meaning.