Halliday & Matthiessen (2014: 261):
In being able to be construed not only with things as participants, but also with acts and facts, ‘relational’ clauses clearly differ from ‘material’ ones; but they resemble ‘mental’ ones in this respect. However, in a relational clause, these things, acts and facts are not construed as a phenomenon of consciousness; rather, they are construed as one element in a relationship of being. Thus while a thing, act or fact construed as a Phenomenon in a ‘mental’ clause is configured with a Senser, in a ‘relational’ clause, a thing, act or fact construed as a participant is configured with another relational participant that has to come from the same domain of being [material vs semiotic].