The following rules are taken from pokemon.com, but they were edited slightly, so make sure to read them even if you are familiar with the VGC 2018 rules.
All battles will be Double Battles played in Pokémon Ultra Sun and Pokémon Ultra Moon
All Pokémon in the National Pokédex are allowed, with the following restrictions:
Duplicate Pokémon are not allowed
All Pokémon are set to Lv. 50, including Pokémon below Lv. 50
A maximum of 2 ubers per team. List of ubers: http://www.smogon.com/dex/sm/formats/uber/
(Nagandel is considered an uber even though it is not currently listed.)
All battles will be best 2/3. You may NOT switch your Pokemon between best 2/3 battles, but you can switch them between rounds.
If you believe your opponent has broken a rule, make sure to contact me about it. (If you save your battle videos, you can record them with a camera and send them to me if you think your opponent cheated. Although it may not be the best quality, it works and it is the easiest way to record a 3DS because obviously not everyone has a 3DS with a capture card built in.
Check the rules regularly because they may be altered eventually.