Markup which is not embedded directly in the text it's marking-up, but which is stored separately and refers to the sections of the original text.
Main advantage is that it doesn't need to be strictly nested. Marked-up spans can overlap without problem.
An example : https://wiki.tei-c.org/index.php/Stand-off_markup
CodexEditor uses it.
Although presumably standoff markups need something embedded in the target text as markers? They just don't need to be block structured?