It’s odd… it appears to be for footnotes. If you enter, for example,
Some text ^[Footnote text], you will see how it works for a footnote in
the preview area. But when you post it, it shows as an inline pop-up like it
does here: Some text [1]
It can be set by the Discourse admins to be either a footnote or an inline
pop-up (which appears to be the case here). I don’t know why it shows as footnote in
the preview but inline pop-up in the post. I guess that’s a bug.
IMHO, it would be useful to have both options available instead of either/or.
Footnote text ↩︎