A document in Glade is not synonymous with an HTML document. A Glade document instead refers to an entire DOM structure that is inside a glade-annotatable tag.
A referent is a given portion of DOM that an annotation refers to. Currently Glade referents are just DOM nodes that are inside a glade-annotatable tag, and are not nested any deeper than that.
An annotation is a piece of content a user creates in the Glade GUI, which is attached to a specific referent, these annotations are created using markdown or plain text.