If this has been previously discussed recently, my apologies. I can find many maps and listings of ATC levels for towers and combination tower/TRACON but am hard pressed to find what defines each level. ARTCC's and combination tower/TRACON are higher levels but metro airport tower/TRACONS differ by a level or two.Why?
It's a complicated formula which combines traffic volume with complexity. Its computation varies depending on type facility (tower, TRACON, up/down, ARTCC, etc.).