Overview
Custom Matchmaking is a Game Mode in Fortnite: Battle Royale. It can be used to play any game mode of Fortnite: Battle Royale but only with those who use the same code. Only those who requested access to the feature from Epic Games, when being a popular streamer/YouTuber etc. can host a Custom Matchmaking Lobby.
It's widely known by content creators for fashion shows, hide-and-seek, and other custom Game Modes where it is not possible in Creative. In competitive, professionals use this for scrims (simulating a Tournament match). Third party tournament organizers use this to host tournaments not in the Compete tab.