Halliday & Matthiessen (1999: 377-8):
Midway between potential and instance, sets of such strategies cluster within ranges of tenor values. Such a cluster is the interpersonal analogue of a domain in the ideation base: it is a region within the overall interpersonal space of meaning, selected according to tenor, just as a domain is a region within the overall ideational space of meaning, selected according to field. The options in interpersonal meaning that make up the cluster together enact a tenor relationship … We might call such a cluster an exchange relationship to foreground that it is semantic (i.e. constituted in meaning through exchanges of meaning) and that it is interpersonal (rather than one-sidedly personal). To indicate that it is analogous to a domain model, we might have called it an exchange or interaction “model”; but we have avoided that term because it suggests a construal of something and construal is the ideational mode of meaning — it is more like a protocol than a model.