Martin (1992: 36):
To this point a model has been outlined in which SPEECH FUNCTION (discourse semantics) has been stratified with respect to MOOD (lexicogrammar) on the content plane. This immediately raises two questions: (i) the determination of speech function in the absence of a one to one correlation between general SPEECH FUNCTION categories and those of MOOD; and (ii) the nature of the units to which speech function is assigned.
 The system of SPEECH FUNCTION is a semantic system, because it is construed as a higher level of symbolic abstraction than the lexicogrammatical system of MOOD. The argument that it is specifically a discourse semantic system has not been made.
 This is not quite right. It is not that there is an "absence of a one to one correlation between general SPEECH FUNCTION categories and those of MOOD", but that, with the exception of offers (giving goods-&-services), there are congruent and metaphorical realisations of (semantic) speech function in (grammatical) mood.
The speech function command (demanding goods-&-services) is congruently realised as imperative mood, question (demanding information) as interrogative mood, and statement (giving information) as declarative mood.
Metaphorical (incongruent) realisations are used to vary tenor, the interpersonal system of context, as are the varying realisations of offer — the speech function that is not grammaticised in the mood system.