If you look here:
http://www.carsearchuk.co.uk/showPicture.php?image=/carimages/i315_5m.jpg
You can see the switch above where B is located. Its for switching between 'Sport', 'Winter' and 'Normal' modes I believe.

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Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i