Identify BOM used several times within other BOM

Hi guys,            

I have the following problem, given this multilevel BOM explosion:

Material A

               l___Material A1

                                       l_____ Material B1

                                       l_____ Material B2

               l___Material A2

               l___Material A1

                                       l_____ Material B1

                                       l_____ Material B2

I need to identify the different usages of material B1 within the whole BOM (let's say I'm going to dissamble material A and I need to know afterwards where each material was). But as the BOM material A1 exists just once, I cannot set an internal value in material B1 position, such value would appear for both usages.

Does anybody know how can I solve this in SAP?

Thanks in advance guys!