Incident sold-to party & product relation

Dear consultants,

We have a requirement for our Solution Manager's service desk. I need the correct approach to the following scenario:

When I select the sold-to party field (e.g BP 142), then I use the advanced search for "Customer Item" is should filter the items of only the BP that I select, but this is NOT what is happening. It's search for all BP's items.

What is the correct approach to do this?

Do I have to modify the BOL object Product, or I have to create a "Z" Search Help?

Thanks & Kind Regards,

César Felce