Probably wrong here, but perhaps T & B are identifying which finger for finger picking ?
Edit.. pretty sure I'm out to lunch here, I was thinking of this..
E|-------------------------------------|
B|-------1----------1------------------|
G|-----0---------0----0------0---------|
D|---2---------0--------2------0-------|
A|-3---------2------------3------2-----|
E|---------3-----------------------3---|
p i m a p i m a a m i p a m i p
edit again.. Since it is up/down strokes maybe T & B is emphasis on treble and bass strings ?