>bracket 1+2
Functionally the same, they're melded together into a thing called Tier C.
No non-land/tribal tutors, including graveyard manipulation like Entomb.
No non-tribal cheat into play effects.
No free spells.
No infinite combos that require less than 10 mana and 3 cards to set up.
Single Target land destruction and non-basic mass land denial (but not destruction).
No 4+ color Commanders.
No Tier B+ Commanders/cards.

>Bracket 3
Now Tier B.
No non-land/tribal tutors. Graveyard manipulation is fine.
No free spells.
No infinite combos that require less than 6 mana to set up.
All forms of land destruction/denial are fine.
No Tier A Commanders/cards.

>Bracket 4/cEDh
Functionally the same thing. Melded together into Tier A.
Everything goes.