i also not sure... but i think 13b15 is like a full bend where as 15b like those quicky types that dun really need to bend all the way one... just og the bending sound can liao...
yea 13b15 is a full bend where you bend the string on the 13th fret to sound like the 15th. 13b could either be a semi or half bend.. most likely its just a quick bend and release like what jayshen mentioned..
