RE: Link Purchase agreement to PO vendor invoice in Pending vendor invoice form
Hello huongpm,
To clarify what I said for "Require Direct Invoicing", a constraint for a PA with a Purchase Agreement Classification is that it cannot be used to release PO. That means that you will not be able to use Direct Invoicing PA to create POs. You will only be able to use them for Vendor Invoices. So they are useful for non-stock items or purchases using categories for which you will likely not have a PO (utilities company, online retailers (?), etc) with whom you have contracts on which you want to exercise some control (amount, enforce maximum, time period,...); the PA can also drive the Financial Dimensions.
For other purchases for which you have a PO (and want a PO), you will use a PA with a classification that is not Direct Invoicing. The PO has to be first created referencing the PA. So you create the PO header linked to the PA and new lines can be added that will draw from the PA agreement (or not if the item/category is not on the PA or if you remove the association to the PA on the PO Line). If PO is linked to a PA, the PA field will appear on the Invoice header when you generate the invoice..

If the association does not exist, it can't be done on the Vendor Invoice. Here is an invoice for PO for same Vendor & item as above that was not created referencing the PA.

Hope that helps!