Comments can be added to items through a combination of automated system comments and user comments manually entered on the item by clicking the Add icon. The following example is on an issue with two user posted comments.
Try the @mention in Comments feature by typing the @ symbol immediately followed by a user's name (no space between the @name). What do you think? Cool, huh.
By default, comments cannot be deleted or edited. More information on setting up comments can be found in the Comment Settings and Comment Types topics.