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 :

Xanadu is meant to use it. (See VisibleLinks)

CodexEditor uses it.

CSS is half like this ... in terms of being specified externally, but matching on classes which still are embedded.(In a sense this is a MiddleSpace between full embedded markup and standoff markup)

Although presumably standoff markups need something embedded in the target text as markers? They just don't need to be block structured?

Not standoff :