I use Platinum.
Clauses are simple rules to keep the game fair and less broken.
Evasion Clause means that moves that raise evasion are banned. This means no Double Team or Minimize. The item Brightpowder and abilities that raises evasion under certain situations are not banned though.
Sleep Clause means you can't put more than one Pokemon from the other team to sleep.
Freeze Clause is related to this. If a Pokemon from the other team is frozen, you cannot put another to sleep.
OHKO Clause means that moves like Sheer Cold are banned.
Soul Dew Clause simply means that Latias cannot hold the item Soul Dew, as it makes her stats too good for Standard play. She is allowed to use it in Ubers though, as well as Latios.
Species Clause simply means no duplicate Pokemon in a team, like, three Salamences are not allowed.