Hi Jaba,
As far as I know, there is no tax code linked to the business partner, but this may be a difference in localization or version between our systems.
Could you please tell me the name of the tax code field in the business partner master data window ?
Please turn on System Information: View > System Information
Now when you see the field, hover over it with the mouse cursor, and check the info field at the bottom left of the B1 client. At the end of that information it will read something like "OCRD.Something"
Regards,
Johan