Halliday & Matthiessen (1999: 154, 155):
Medium and Process form the nucleus of the whole figure — that part of the figure which is essential to the complementarity of unfolding and persisting. […] The model thus construes a nuclear figure consisting of a process unfolding through the medium of a participant. […] It is in the combination of Medium + Process that we find the complementarity between the temporal unfolding (the Process) and the atemporal persistence (the Medium).