Halliday & Matthiessen (2004: 480):
It is still a meaning, a semantic abstraction, not some third type differing both from meanings and from wordings (indeed there is no third level to which it could belong). But it is not a meaning created in anybody’s consciousness, nor is it emitted by any signal source; it is simply got up so as to function as a participant in some other process — typically a relational process, but sometimes also a mental or verbal one.