Usually the way I would interpret that notation just as you said a release bend from the specific note at the 15th fret down to most likely the originally fretted note at the 13th fret. If it's just written "15r13" I would fret at the 13th fret and bend up a whole step THEN PICK the string and release the bend back down to the 15th fret. This gives you a nice Steel-Pedal-Guitar-sound to your bends and is a nice Blues tool. However, it says "13b15r13" Then it's Pluck, bend, release. That's the way I would interpret that but almost every tab you find on the internet will be slightly different because everyone has different ways of playing stuff... so listen to the song and see how it sounds to you.