Halliday & Matthiessen (1999: 524-5):
If the commodity being exchanged is goods–&–services, then the action that is given or demanded is typically a non-verbal one: what is being exchanged is something other than a construction of meanings, and the meanings serve to bring the exchange about. … If on the other hand the commodity being exchanged is information, then this is in fact made of meaning; the speaker’s action, and that of the listener if responding to a question, is bound to be a verbal one, because here language is not only the means of carrying out the exchange, it is also the nature of the exchange itself.