There is the legend on the upper-right corner explaining how tiers are assigned and that's exactly what s00pahFr0g said: "A" tiers are necessary in one of the 3 Epic Arena teams; "B" tiers need the right setup; "C" need the right setup and opponents(Leah is great with Tholin, Tareth, etc but she even needs the right opponents to shine...obviously vs Vincent, Mortus, Gizmo, FH and Dokras she's the best to use...still situational!); only "D" tiers are bad.
The old tiers were: "A" = Zem, great heroes, good, great situational; "B" = quite bad, still viable; "C" = useless/not a spot in the meta; "D" = Esabel.
@KemosabeTBC You still reply based on the old rates: you say "this hero is situational and he has C"; if you see the lenged, "C" means situational!
@Namorat considering the current meta, "is he a must in one of the 3 teams in Epic Arena?" We may consider this the question for A ratings...