Halliday & Matthiessen (1999: 266-7):
Relators show the least organisation of any, since they are experienced indirectly in the form of logical relations between other configurations; they share some of the systematic features of minor processes, but other than that they display only the contrast between the two relative statuses they assign to these configurations, as being equal or unequal (paratactic / hypotactic in the grammar) — a then x / x after a // b so y / y because b &c. Thus a relator can be metaphorically reconstrued into any other category … whereas a process can be construed only as a participant (quality or thing), and a quality only as a thing.