Halliday & Matthiessen (2014: 214):
In addition to material and mental processes — the outer and inner aspects of our experience, a third component has to be supplied, before this can become a coherent theory of experience. We learn to generalise — to relate one fragment of experience to another in some kind of taxonomic relationship: this is the same as that, this is a kind of the other. Here the grammar recognises processes of a third type, those of identifying and classifying; we call these relational process clauses …