Good question.

I guess we could consider Tier 1 to be megahubs (ATL, DFW, MIA, etc.), tier 2 to be large cities that are huge (BNA, RDU, SLC, STL, etc.) and tier 3 to be smaller airports, still with mainline service (MHT, PVD, ALB, etc.). Tier 4 would be larger regionals (PWM maybe?) and tier 5 would be the rest (GNV, etc.). Tier 6 could be those cities with a few EAS props a day.

Then again, I am just making up my own definitions here.