In a doc sent me by an outside party there's a mark I can't identify.

It appears in headings, using the built-in Heading 2 style, but modified.
The mark looks like (two greater than signs), only smaller and a single
character. It's followed by a paragraph mark. Also, it appears to be red.

When reveal codes is on, the heading appears to end after the paragraph
mark. However, when reveal codes is off, the text appears to continue on the
same line as the heading text. Also, in the TOC, only the heading text
before the mark is picked up.

It seems like a useful little trick, but how do you duplicate it?