When the [help center](http://emacs.stackexchange.com/help) doesn't have the information you seek, check the [main Stack Exchange meta site](http://meta.stackexchange.com/), starting with [questions tagged FAQ](http://meta.stackexchange.com/tags/faq). There is an [FAQ on comment @replies](http://meta.stackexchange.com/questions/43019/how-do-comment-replies-work).

The main rules are:

* @-replies only work in comments. If you type `@` in a post, it's just an ordinary character, there's no way to notify a user via a post's content. (Also in chat with different rules that I won't discuss here.)
* You can only @-reply to a previous comment in the same post, to the poster, or to an editor of the post (plus a few more edge cases, see the FAQ for the full story).
* You can only have at most one @-reply in a comment.
* The author of the post you commented on is always notified, in addition to the user you @-addressed.
* Sometimes your @-mention will be erased when you post the comment. Don't worry, that only happens if that user would have been notified anyway without the @-mention.
* To notify a user, type `@` and the first letter of their username, and let completion happen. If you want to type the name, it's the full name without spaces, or a prefix of at least 3 letters. See the FAQ for the nitty-gritty on abbreviations when some users have similar names.