Anything goes was created in lieu of Mega Rayquaza's ban from the Ubers tier. You are permitted to use virtually any Pokemon, move, and ability you want. You can't give a Pokémon a move it normally doesn't learn though. However you can use moves that are banned in the other tiers, such as OHKO moves, evasive moves (double team/minimize). You can have six of the same Pokémon, use items that are normally banned, such as Soul Dew with Latios.
I believe the only restriction in place is the Endless Battle clause.