It's important to differentiate between BB/100 and bb/100
The first, originally meant 'poker tracker big bets/100 hands'
the second meant 'big blinds/100 hands'
Poker Tracker 4 lists the stat BB/100, but strangely it's actually bb/100 that the stat reflects.
The original version of BB/100 (or PTBB/100) means that at a value of 5, you are winning one buy in per 1000 hands
Another thing to note is that the concept of 'big bet' comes from limit poker, but the stat BB/100 is one used by big bet players too.