Custom Match Making is a feature that allows players to set up private matches, where only other players who they invite can join. The players who decides to create a custom match will be able to make their own key code, which they can then share with the other people they are hoping to join them. Only players who receive the same key code will be allowed to play in the match, which they will need to enter said code in the Custom Match Making Section of the Main Lobby (bottom right corner) in order to do so.
Custom Matches are great for larger groups of friends that are looking to pay with one another, as well if you're llooking to play certain matches under specific, custom rules. Should you want the participants of your custom match to only be allowed to use a certain item, you can definitely do that with this feature. Shoot, you can even make it a requirement for all players of your custom match to play in a certain skin or outfit! On top of all of this, Custom Matches can even be set up with tournament-style play!