Difference between Trading partner and Payee columns appeared in PPR @ Invoice pending review status

Hi All ,

 About the issue :

---------------------------------

1)  In PPR @ Invoice pending review status trying to click on Add scheduled payments however not getting list of values under any criteria.

 The related screen shot uploaded in to post .

Indetails -

I can remove the invoices by selecting then by clicking on Add schedule payments i need to add one invoice however any of the criteria iam unable to get the information to add

2) In this stage only  (observe in that screen ) both Trading partner and Payee columns dispalying ... however both are same ,

please let me now what is the difference between those two , if same how (need a valied justification)