Halliday & Matthiessen (1999: 514):
… these [verbal processes] shade into something else, which the grammar again construes as phenomenally distinct: relations of identity (including symbolic identity, like red means stop) and attribution. Expressions of this kind, which in English often have the verb be, hardly seem to fit the label “process” at all; but the grammar firmly represents them as such, so we call them “relational processes”.