Today I brought up the Pokemon XD rule menu in the Deoxys thread, and it doesn't seem like a lot of people know about it. Its options aren't even mentioned on Bulbapedia. I recorded a quick clip going through it for DaWoblefet and ended up making a slightly more detailed video shown here. Personally I have not actually played XD's battle mode and I'm not sure how well Dolphin even works with it, so there are no mechanic tests, just scrolling through the menu itself to show its options.
By default you have 6 rules, with 3 presets of "Anything Goes", "Max. Lv 50", and "Max. Lv 100" along with 3 editable rule slots. Apparently, Pokemon Colosseum also has a similar rule editing feature according to the Pokemon Colosseum manual
On page 1 you can set level range restrictions, as well as add a combined level limit, like the cups from Stadium had. There are also settings for a forced number of Pokemon and time limits per battle/move selection, counted by minutes and seconds respectively. Again I don't actually know how any of this plays out ingame, but for some mechanics, we can refer to the Colosseum manual (page 45). Apparently the player with the fewest fainted mons will win, and if they're the same it compares HP totals to determine a winner. It also says running out of time will automatically select the first move slot, just like the main games do.
On page 2 there's bans like Species Clause and Item Clause, as well as the ability to turn on hold items. If you press A on "Restricted", you get to pick and choose all the hold items you want allowed, with Soul Dew off by default. I noticed that for some reason Cleanse Tag is in this menu despite having no battle effect.
Page 3 contains rule mods like revealing Deoxys formes (referred to as disabling DEOXYS's camouflage ability), the classic Sleep Clause, and also disabling Skill Swap for some reason. I am not sure if this makes it fail or just bans it, but I'd assume fail. I also don't know what you get if you trade a Deoxys to XD, but I suppose it must have the data and models for each form if it has this rule. While looking up "Deoxys's camouflage ability", I found a guy on serebii forums from 2005
saying that a Deoxys from FireRed looked like Normal forme but with Attack forme stats. As a sidenote, while I was writing this post Plague von Karma pointed out in the Deoxys thread
that Sleep Clause counts Rest in XD.
Page 4 is the place for basically blocking "cheap" strategies. There's self-KO clauses for losing with Explosion/Selfdestruct and having Destiny Bond/Perish Song fail when used by the last mon. This page also has Freeze Clause and lets you make Dragon Rage/SonicBoom fail for Little Cup.
Finally, page 5 is just where you can reset the rules to defaults. When you're looking at a preset rule there isn't even a page 5, so technically there are only really 4 pages for rules.
So that's the Pokemon XD rule menu. I don't think this will be that relevant to competitive, but I think it is neat and it's documented now.