If you have the admin rights to do so, you can adjust whether giving a Reason for Rejection and Comments to Requestor are mandatory when rejecting an invoice.

In TC 3.3 SP2:

1) In the Admin Tab, under Admin Settings, navigate to the General Page:

2) Then under the "Invoice Rejection Settings" block seen above, you can check or uncheck "Reason for Rejection is required" and "Comments to Requestor is required" in order to adjust how much information is given for invoice rejections. If both are checked, then when rejecting an invoice request, on the "Confirm Rejection" popup, a dropdown menu for "Rejection Reason" and a "Comment to Requestor" field will need to be filled out by the rejecter before a rejection can go through, as seen in the screenshot below. 

In this way, feedback is provided on how to improve invoices before they will be accepted.

In TC 3.3 SP3:

1) In the Admin Tab, under Admin Settings, navigate to the Invoices Page:

2) Then under the "Invoice Rejection Settings" block seen above, you can check or uncheck "Reason for Rejection is required" and "Comments to Requestor is required" in order to adjust how much information is given for invoice rejections. If both are checked, then when rejecting an invoice request, on the "Confirm Rejection" popup, a dropdown menu for "Rejection Reason" and a "Comment to Requestor" field will need to be filled out by the rejecter before a rejection can go through, as seen in the screenshot below. 

In this way, feedback is provided on how to improve invoices before they will be accepted.