It's not the game that's custom, it's what you bring. So, via import export, you can set your pokemon to level 9999, give them 24 moves, any ability, all evs, and any item. You can also use formes from the start, like just clicking Mega Rayquaza or Darmanitan-Zen. Unlike Balanced Hackmons, which is a metagame, Custom Game is for testing; thus, everything is legal, including abilities such as Wonder Guard a sim-only move called Magikarp's Revenge. You may also find that people use custom game to play certain <other metagames> not available on the main server.