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,
such as that between a client and a server (unequal in power, low in familiarity, neutral in affect) where the client initiates demands for goods-&-services and information about them and the server responds,such as that between two friends (equal in power, high in familiarity, positive in affect) where the interpersonal options are wide-ranging, orsuch as that between the writer of a weather report and the readers (unequal in expertise, no familiarity, neutral in affect) where the writer gives information and the readers accept.
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 like a model.