Business Partner in the Role of Plant BUR006

In SRM 4.0 (and probably in CRM as well) the plant is also a business partner via BUR006 relationship. Plant is assigned to company code in the extended attribute section, but in contrast to BP in the role of employee BUP003 I can't find the table holding such an assignment or a field within transaction BP. Does anyone know where this attribute's relationship is stored outside of transaction PPOSA_BBP?