Halliday & Matthiessen (1999: 441-2):
Throughout our discussion of the organisation of the meaning base, we have made reference both to intra-stratal considerations (such as patterns of agnation and the transphenomenal types that emerged in the course of our exploration) and to inter-stratal considerations. With respect to the latter, we have foregrounded considerations 'from below', from the stratum of lexicogrammar.There were two main reasons for this: on the one hand, the meaning base has to be realised in worded texts and the statements of realisation will be simpler if the resources of wording are part of the picture from the start; on the other hand, the relationship between meaning and wording, between the system of semantics and that of lexicogrammar, is a natural one: they are both strata of the content.Although it has not been part of the central argument about particular distinctions in the meaning base, we have also referred to the relationship between semantics and context. Just as the meaning base has to be accountable lexicogrammatically, it also has to be accountable contextually. In our discussion, we emphasised register or functional variation as one of the keys to the relationship between semantics and context.