[Register] is set of meanings, the configuration of semantic patterns, that are typically drawn upon under the specified conditions, along with the w… - Michael Halliday

p. xiv cited in: Piet Van de Craen (2007) Van Brussel gesproken. p. 118.

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

It is part of the task of linguistics to describe texts, and all texts, including those prose or verse, which fall within any definition of literature and are accessible to analysis by the existing methods of linguistics.