How to use Amount correctoins functionality in R12.1.3

Hi Can you clarify how to use Amount correctoins functionality in R12.1.3

I know that :

Query the invoice or purchase order that you want to correct. Click Find.
The window displays the available purchase order shipment lines based on your correction type and query.
The type of correction that you can make depends on the type of purchase order line you are correcting:
. Goods. You can make price or quantity corrections.
. Services. You can make amount corrections on all services, with the exception of Rate Services.
. Rate Services. You can only make Quantity corrections for Rate Services.


But this is not working for Service based PO's Amount correction