Replying and Messaging
You can send a notification to another user and draw their attention to a comment, without replying to one of their topics or comments. To do so, type @username in the comment. /u/username and u/username will also work. This will send a notification to the other user's inbox. There is no limit on the number of users you can tag in a single comment.
User notifications do not work in topics, only in comments.
Reference: Username mentions in comments now send notifications