Update: Material below the (old) marker is incorrect. I'm going to go live in a cave and never touch a computer again, but before I pack my bags, here's the real scoop.

A person mention is the act of linking to someone's homepage when mentioning them in post content. It's worth having a special name to distinguish homepage links because it's a common IndieWeb practice to handle WebMentions sent to a homepage differently from those sent to other paths on the site.

A person tag is the act of marking a post to indicate someone is involved in the post in some way. Examples: the person was present in a photo being posted; the person was hanging out with the author when the post was made. A person tag requires a person's homepage link (which makes it a type of person mention), as well as the u-category microformat which indicates a post is "about" some topic (in this case, a person.) Finally, person tags are usually added through an interface which complements the posting interface. Consider the difference between @-mentioning someone in a Facebook post and tagging them in a photo.

(old)

On the IndieWeb developer chat, we've been discussing the difference between "person tags" and "person mentions." There's a difference, but I don't think the wiki page correctly describes that difference.

The problem with the IndieWeb wiki page for person-tag lies in the very first line, which currently says that a person tag is a person mention. However, the rest of that page, and the page for person mentions, does not support the claim that a person tag "is-a" person mention.

A person tag is an HTML chunk that forms a reference to someone, by being marked up with specific HTML attributes (class="u-category h-card" and href="homepage.example.org"). A person mention is a Webmention sent to a specific person's homepage to notify them of a post which in some way is about them/concerns them. A reference and a Webmention aren't the same thing: that's problem number one for the above quote from the person tag page.

Problem number two is that the person mention page is explicit that as long as the content at the Webmention source is relevant to Mister Example, and contains a hyperlink to his homepage, mr.example.org, then a Webmention sent with that source and target=mr.example.org is a person-mention: if you're doing something beyond merely mentioning someone, e.g. inviting them to an event, or person-tagging them, then see [the "invite" and "person-tag" pages] for how to add that markup (source).

You can put a person tag in a post without sending a Webmention¹ for that person tag: maybe you just want to use people's names as tags on your site without notifying them of it. And you can send a person mention without having a person tag in the post, as described in the "merely mentioning" quote above.

I had trouble understanding the distinction because I can't think of a practical scenario where I would want to mention someone in the written content of a post, and send them a Webmention about it, without also wanting to explicitly indicate that I'm "tagging" them (and taking the extra step of adding person-tag markup to the post.) On the other hand, I can imagine person-tagging my posts without sending person mentions to let people know about the tags.

[Thanks to several people for helping me understand these concepts, including Tantek, Jacky, and Malcolm.]