If the "Mandatory Rejection Comments" feature flag is ticked as per instruction here, this feature will be activated immediately for not closed existing forms with unfinished approval sections and new forms with approval sections.
This means, when a form approver clicks Reject button for a form approval section without entering comments in Commentary field, the system will:
Highlight the Commentary field label as red text.
Show a red warning dialogue “Please complete all required fields“.
This will remind the form approver to provide some comments before rejecting an approval request, so that the approval requester can gain a better understanding for the reason of rejection.