A mononuclear relation holds a nucleus and a satellite, where the nucleus expresses the main textual information and the satellite offers additional information about the nucleus (Stede, 2008), while a multinuclear relation holds two or more discourse units, which are all nuclei.
three types of nuclearity exist: Nucleus-Satellite if the left subtree is the nucleus and the right subtree is the satellite, Satellite-Nucleus if the order of the satellite and nucleus is inverted, and Nucleus-Nucleus for multinuclear relations.