Martin (1992: 76):
[The discussion] has dealt only minimally with intonation; whether this is treated as a meaning making resource in its own right following El-Menoufy (1988), as most delicate MOOD following Halliday (1967c), or as a direct coding of discourse structure following Brazil (1981), this omission is a serious one.
 This again confuses semogenesis ('meaning making resource') with stratification ([realising] 'most delicate mood', 'coding of discourse structure').
 Halliday does not treat intonation as most delicate mood. Intonation is a system in the rank scale of the phonological stratum (expression plane), whereas mood is a system of the lexicogrammatical stratum (content plane).
The interpersonal intonational system of tone realises the interpersonal lexicogrammatical system of key (more delicate mood), while the other intonational systems, tonicity and tonality, are resources of the textual metafunction.
Halliday & Matthiessen (2004: 142):
The tones are not, however, simply additional markers attached to the realisation of mood. They realise distinct grammatical systems of their own, which are associated with the mood categories. The general name for systems that are realised by tone is key.