discourse relation is used to link discourse markers. These markers are not clearly linked to the structure of the sentence, except in an expressive way.
In some cases the discourse marker can be another proposition. Ex: He comes on wednesday, I think. In this sentence I think is a discourse marker because it lacks an object (the object of the verb think is the rest of the sentence) and because they are fixed (I think doesn’t commute with he thinks).
discourse relation is also used for simple discourse markers such as interjections or adverbs.