It's for something that is either missing, or the car was not equipped with. Could be SDARS, or TCU, or SVS, or something else. The fiber optic jumper is installed to keep the MOST bus ring in-tact.
If what I'm speaking sounds like Greek to you, then you might just want to leave it alone. Unless there's a problem you are trying to fix. In that case, you'll need to post more info about what you're trying to accomplish. Either that or seek the help of a professional.
ASE and BMW Master Certified Technician
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