Each form submission/entry has it's own comment section. If you can see the submission, you can comment on it.

This means that the person who filled out the form or lodged the submission (except on a public form) can see any comments, and Admins can always see comments as well. Any other users/roles who can see it, will be determined by the form permissions.

Other users are able to be tagged (and notified of the tag) on the comment section. You can only tag as user who has permission to see the submission.