Non PO Spend by Category report

Dear experts,

I would like to clarify the following. We have 'Non PO Spend by Category' report in SPM 3.0. It compares the total spend vs spend without PO reference. My question is, since invoice which does not have PO reference cannot be linked to a category, how is the non-po invoices identified for a category?

Could someone please provide an explanation?

Thank you for your help.

Best regards,

Madhur Shah