Halliday & Matthiessen (1999: 378):
A particular domain model within the ideation base correlates with a particular exchange relationship within the interaction base and together they form the ideational and interpersonal aspects of a register. An exchange relationship thus has ideational implications: it involves the exchange of some ideational meanings rather than others and it embodies a division of labour between the interactants in an exchange relationship. … An exchange relationship thus gives interpersonal values to meanings within the domain model it is associated with.