For example, a document is a scanned PDF of a proof of payment for $200.
This document is uploaded to support a partial payment made to a $1000 expense transaction. 
When the proof of payment is uploaded, the user would enter $200 in this new Transaction Value field. 
There might be a few different documents attached to one transaction. These represent supporting documentation for each payment.
The sum of these fields for one transaction, which corresponds to the sum of all documented payments to that one transaction is now available within the Grant Transaction reports.