General

Rare (0-30)
Epic (31-70)
Unique (71-120)
Legendary (121+)

there is actualy a whole guide about this on basil. go to items>Misc>Potential & Cubes

Edit: incase the tier is confusing, just because the level tier is called "Rare (0-30)" doesnt mean you cant get epic potential on a level 0-30 item.

also guys, should i cube my empress or some regular equips such as osfa and pirate. i plan on playing other thief classes and empress is just too expensive. any advice? i have 6set (with dagger)

Tier 1:Lvl0-30 item Gives out 1%, 2%, or 3% stats.
Tier 2:Lvl31-70 item Gives out 2%, 4%, or 6% stats.
Tier 3:Lvl71+ items Gives out 3%, 6%, or 9% stats.

At legendary, the Lvl71+ equips can get 12% stats.

0-30 = first tier
31-70 = second tier
71+ = third tier

Also note, weapons and secondary weapons can only get 35%/40% Boss/Ignore if they're level 100+.

there is rare, epic, unique and legendary. they are from worst to best respectively. and each tier gives better amount of stats for example rare would give 3% str per line at maximum whereas epic would give 4%(dont quote me on that), unique would give 9 and legendary giving 12.
For equips getting "tiers" they get better potentials depending on what level the item is. i think its tier 1(lv 1-30) tier 2(31-80) tier 3(81+) but i dont remember very well. for example. tier 1 cannot get % boss dmg whereas tier 3 can get it. that made no sense but whatever i typed it anyways

@Aaaayee both i really know nothing about tiers and what gives what

Level potential tiers or just potential tiers themselves?