With the match date and the match time you offer your opponent a certain point in time. When your opponent accepts this point in time, you an open challenge!
When using the fast challenger you send your opponent a challenge with a limited validity. If the challenge is not accepted within this frame of time, the challenge expires.
In case of a challenge at least 2 maps have to be declared or choose random map. If your opponent chooses random map aswell the system chooses the map to play. In case of chosen maps your opponent is allowed to choose which of the 2 maps he wants to play against you. Your opponent is also allowed to choose totally different maps. Then the challenge comes back to you and you have to decide which map is played.