Can anyone tell me the state a Purchase Order has to be in to be queried in the Find AP and PO Write

Can anyone tell me the state a Purchase Order has to be in to be queried in the Find AP and PO Write-Off Candidates window please?

I am trying to understand under what circumstances a PO can be queried in this form.

In test I created a PO and fully receipted it but created invoice for lesser value and matched to PO. The PO was not available in this form (CSTACRAP)  

Are there any good documents that explain the AP and PO Accrual Write-Off window functionality?

Any help much appreciated,

Thanks Rodders.