The Playing Efficiency is a measure of how efficient the count's associated indices for altering your plays from Basic Strategy are.
The greater the number of decks in the game the less important this is. Ergo, in a hand-held ("pitch") game, a High P.E. (above .60) is crucial
The Betting Correlation is a measure of how efficiently the count is directing your bet-sizing. Ergo, in a shoe game a High B.E. (above .97) is crucial.
There is also an Insurance Correlation, that is an important metric to be considered.