ptbb/100 is synonymous with BB/100

Technically it shouldn't be written as ptbb/100 as everyone generally accepts BB=big bet and bb=big blind,

So ptbb/100 doesn't really make much sense, it should be ptBB/100, or simply BB/100 !

Edit: oh I see spendas written ptbb/100 in which case I'm obviously wrong!

Anyway : -

bb=big blind,
BB=big bet (from limit where the big bet, was two big blinds)
ptbb/100 = poker tracker big bets per one hundred hands.