It's just to allow users to have more than 12 available to trigger at a time. Currently, on the Presets window you can MIDI map the buttons 1 to 12 to trigger the current bank of presets. The "B2 P1" mapping stands for "Bank 2, Preset 1" - so, it'll trigger the first preset from the second bank (though "second bank" is relative to the current bank, so if on the preset window you have bank 4 selected, then B2 P1 will trigger the 1st preset from bank 5).
