Halliday & Matthiessen (1999: 113):
Interpersonal semantics is centrally concerned with varieties of symbolic exchange. Here, as in other places in the meaning base, the system is organised in such a way that it creates a difference between non-symbolic reality and symbolic reality, between phenomena and metaphenomena. The "commodity" that is being exchanged in interpersonal dialogue is either semiotic or material: it is either one that is construed by language itself — information — or it is one that exists independently of language — goods & services. In the first case, language constitutes the exchange; in the second, it facilitates the exchange of a nonlinguistic commodity. For example:
