Halliday & Matthiessen (2014: 135):
If you say something to me with the aim of getting me to do something for you, such as ‘kiss me!’ or ‘get out of my daylight!’, or to give you some object, as in ‘pass the salt!’, the exchange commodity is strictly nonverbal: what is being demanded is an object or an action, and language is brought in to help the process along. This is an exchange of goods–&–services.