Comments can be attached to completed events as a way to capture and record a discussion outside of the defined event workflow.
The functionality to attach a comment can be found at the bottom of every completed event, as shown below.
It will display summary information on the number of tags, documents and comments and clicking show more will open up the ability to write and save a comment.
Clicking save will attach this comment to the event. This will send a notification email to the owner of the event to inform them that a comment has been posted on their event.
The owner can then choose to post their own comment which will send a notification email to all other users who have previously posted comments to this same event. In this way the comments functionality can generate a discussion on any given event.
If a user has the permission to view an event, either their own or that of someone else, then it is always possible for them to attach a comment. There are permissions available if you would like to give the user the ability to delete comments.