Purchase Order and Invoice relationship

Hi all,

I'm trying to find the relationship between Post Purchase Invoices and Purchase Orders.

The purchase invoices are NOT posted from the purchase order, rather created by a separate Purchase Invoice and using Functions, Get Receipt Lines

I do have a Pre-Assigned No. (PI) but NO Order No.'s (PO) but don't know how this relates back

Example assigned Series #

PPI000123 - Posted Purchase Invoice

PI0000123 - Pre-Assigned No.

PO400012 - Purchase Order

This is to amend the Detailed Trial Balance Report to show the original Purchase Order or Sales Order