Why Bill-to BU in the BPA Controls tab can't select a BU which is different with req BU?

Summary:

Why Bill-to BU in the BPA Controls tab can't select a BU which is different with req BU?

Content (required):

We active the financial orchestration which makes us to create a PR/PO in different req BU and pro BU. For example, Req BU is USA and Pro BU is UK. In that way, we should be able to select UK as the bill to BU and USA as the requisition BU in the blanket agreement Controls tab. However, the situation is we can't. Req BU and Bill-to BU has to be the same- both UK or both USA.