Both BB/100 and bb/100 are metrics of a player's winrate per 100 hands. In PokerTracker 4 they are defined by the game type being played:
NL/PL = BB is Big Blinds
Limit = BB is Big Bets, and bb is big blinds*
So if you are playing $1/$2 NL and check your stats:
This means that your winrate is 9.08 BB/100, and that you are winning $18.16 per 100 hands.
The same thing holds true when you check out stats like All-in Equity Adjusted BB/100. In NL/PL games it refers to big blinds per 100 hands, and in limit it refers to big bets per 100 hands. PokerTracker 4 automatically knows which game type you are playing and shows the results using the correct BB/100.
* Currently the only place where PT4 shows bb in a limit game is in the Live BB statistic.