Halliday & Matthiessen (2014: 25):
We use language to make sense of our experience, and to carry out our interactions with other people. This means that the grammar has to interface with what goes on outside language: with the happenings and conditions of the world, and with the social processes we engage in. But at the same time it has to organise the construal of experience, and the enactment of social processes, so that they can be transformed into wording. The way it does this is by splitting the task into two. In step one, the interfacing part, experience and interpersonal relationships are transformed into meaning; this is the stratum of semantics. In step two, the meaning is further transformed into wording; this is the stratum of lexicogrammar. This is, of course, expressing it from the point of view of a speaker, or writer; for a listener, or reader, the steps are the other way round.