Martin (1992: 58-9):
Halliday's (1985) notion of congruence bears on the problems encountered here. Just as one can argue that there is an unmarked relationship between grammar and phonology whereby tone groups are associated with a single clause, so one might suggest that a similar unmarked relationship holds between a move and a clause complex: generally speaking a move in the exchange will be realised by a clause and its dependents.
This is not congruence. In SFL theory, congruence refers to a non-metaphorical relation of meaning (semantics) to wording (lexicogrammar).
On the other hand, the unmarked option is 'the form we tend to use if there is no prior context leading up to it, and no positive reason for choosing anything else' (Halliday & Matthiessen 2004: 58), in contradistinction to marked, which means that the option is less frequent and 'carries a special interpretation' (Halliday & Matthiessen 2004: 207).
The realisation of a single clause as a single tone group is unmarked tonality — where tonality refers to the selection of the number and boundaries of tone groups (Halliday 1970).