Comments allow you quickly collaborate on a document with other users.
From search results, hover over the vertical dots on the right side of the document card and select comments. Type the @ symbol from within a comment box to reveal a list of the users in your project from which you can choose. For instance, if you type @ja you'll see users that have the letters JA in their names, like Jane Doe.
You can do the same from the document viewer:
Once you select the person's name from the list, they will be listed in the comment and receive an in-app notification that you mentioned them. If they have the option enabled in their personal preferences, they will also get an email notification. The email notification will contain the entire document-comment thread (not the document itself) and a link to open the specific document in their browser. They can then reply to the comment by email without being logged into Logikcull. It looks like this: