Martin (1992: 33):
This is an important step as far as interpreting dialogue is concerned since there is no simple relation between general grammatical classes and the rôle they play in structuring a conversation. The most commonly discussed example of this is the realisation of demands for goods and services. Each class considered so far, for example imperative, declarative and interrogative MOOD, can be used to code a Command.
 Trivial point: imperative, declarative and interrogative MOOD are grammatical functions, features, not grammatical classes.
 Any rôle that a grammatical system has in structuring a text such as a conversation is a resource of the textual metafunction. Speech functions, such as commands, and the moods that realise them grammatically — either congruently (imperative) or metaphorically (declarative and interrogative) — are resources of the interpersonal metafunction. Here the two distinct metafunctions are confused. The interpersonal metafunction is concerned with enacting relationships, the textual metafunction is concerned with organising the interpersonal and ideational into texts.