How can I make sure to collect a comment when a form submission is closed?

Make sure you understand why a form submission was closed or rejected with the Comment On Close feature.

The Comment On Close feature allows administrators to force the user closing a form submission in the In Progress section to add a comment. This way, administrators can know what actions were taken to close, or even reject, a form submission. 

Where can I see these comments? 

These comments are visible in the Closed table in the In Progress section, in the Show Details side panel, as well as a column in Form Reports. 

How can I enable this feature? 

Easily enable this feature for your form by checking the "Requires a comment when closing the ticket" checkbox in Form Settings.