Difference between the statuses - "APPROVED" and "WFAPPROVED" in AP_INVOICES_ALL

Hi,

In AP_INVOICES_ALL table, the column wfapproval_status have values as "WFAPPROVED" as well as "APPROVED".

When we have this value as "WFAPPROVED" ( in application it will show as "Workflow Approved'), then we try to make a payment, ( through Actions--> Pay in Full option), we can see the Invoice amount got defaulted as "Payment Amount".

However when the value is just "APPROVED"( in application it will show as "Approved'), then the while making the payment, the "Payment Amount" is getting shown as 0.00.

Why it is so? And what will be the difference between these 2 statuses - "APPROVED" and