Writing @handle in a post asks for that post to appear on their page as well as yours. It is for the photo of the two of you, the night you were both at, the thing you made together.
How it goes
- It appears on your page straight away.
- It appears on theirs only once they say yes. They get a notification reading wants to post this on your page, with the post quoted, and three choices: Put it on my page, No thanks, and Read it first →.
- You are told either way. Declining deletes the request rather than leaving it sitting there.
- Until they answer, nobody but the two of you knows the request exists. A decision somebody has not made yet is not news.
What it changes about who can see it
Both of you agreed to that — you by tagging, them by accepting — and it is the point of the post being on their page at all. It is also why a Only me post cannot tag anyone: the audience there is nobody, so there is nothing to share.
What gets refused, and why you will know
You can tag up to five people, and only people who have accepted you as a friend. Tagging someone who is not a friend, or tagging on a private post, refuses the whole post and names the problem. It is refused rather than quietly dropped on purpose: somebody who writes @kenzie, sees no error, and assumes it worked would find out weeks later that it never reached her.
sam@example.com is not a mention. The @ has to start the word.
Fixing a typo in a post later does not re-ask anyone who already agreed, and does not notify them twice. Taking a handle out of the text removes the tag.