How is BPA Line sourced onto Manual PO (no backing Req)

Summary:

When creating a manual PO (no backing REQ) after entering the Item the UOM is defaulting from BPA. BPA has 2 different UOMs for the same item.

How does Procurement decide which BPA line to select?

Content (please ensure you mask any confidential information):

BPA123 has item A with 2 different UOMs (CA and BG) at 2 different price points.

line 1 Item A UOM: BG Price 1.00

Line 2 Item A UOM: CA Price 20.00

When creating a manual PO (No backing req) after entering the Item on the PO line, the UOM is defaulting in as BG. I want to manually change the UOM to CA. The UOM field is not editable and the PO line is tied to BPA Line 1.