I read in a comment that users can notify each other by starting the comment with
- Assume I want to notify Marco Ceppi. Do I write
- What does that user see? How is this notification shown?
Only applies to other people in the comments that you are commenting on, or editors of the current post.
Response must include @username that you are referring to, where "username" is a reasonable match to the user's current display name (as seen in the comments above yours).
There must be a starts-with, case insensitive match of at least THREE characters to the displayname. So @a and @ab will never match anyone or anything.
Spaces cannot be used to match, so if the person's display name is "Peter Smith" then just use @peter or @petersmith to match.
Matching is performed in reverse chronological order, so if there are five people named "John" in the comments, writing "hey @john, have you considered apples?" will match the most recent John to comment.
To answer my question #2 (based on fluteflute's helpful demonstration) the notification consists of an update in the "StackExchange" icon/menu in the top left of the page.
There's a comprehensive (and updated from Jorge's) list at meta.stackoverflow.com. I'm not quoting it here so anyone won't be tempted to not read it and thus miss updates... and so I don't have to keep this updated myself. :)