They just make up designations at random (fairly)...
There is no link to the engine type, for example the B6 3.5 and B10 3.5 had the same engine. So did the B8 4.6 and the B10 V8...the only real rule of thumb is that the lower numbers usually refer to the 2002/3 series (eg A1, A2, A3, A4, B3, B6, B8, C1, C2), the medium numbers to the 5/6 series (eg B7, B9, B10, D10) and the highest numbers to the 7/8 series (eg B11, B12)
/Kees