In the relative orientation of different social groups towards the various functions of language in given contexts and towards the different areas of… - Michael Halliday

A physical system is just that: a physical system. What is systematized is matter itself, and the processes in which the system is realized are also material. But a biological system is more complex: it is both biological and physical — it is matter with the added component of life; and a social system is more complex still: it is physical, and biological, with the added component of social order, or value. … A semiotic system is still one step further in complexity: it is physical, and biological, and social —and also semiotic: what is being systematized is meaning. In evolutionary terms, it is a system of the fourth order of complexity

[A register is constituted by] the linguistic features which are typically associated with a configuration of situational features - with particular values of the field, mode and tenor.

I see it as part of the development of the field. I would always emphasize how much I share with other linguists: I've never either felt particularly distinct or wanted to be distinct. I never saw myself as a theorist; I only became interested in theory, in the first place, because, in the theoretical approaches that I had access to, I didn't find certain areas developed enough to enable me to explore the questions that I was interested in.