Halliday & Matthiessen (1999: 321):
Together, field, tenor and mode define the ‘ecological matrix’ in which particular types of text are processed: there is a systematic relationship between such matrices (particular combinations of field, tenor and mode values) and particular types of text. We can see this clearly with actual instances of text — for example, an individual recipe or weather forecast; but these are instances of general classes, to be characterised in terms of the systemic potential that is instantiated in them. That is, there is a correlation not only between a contextual matrix and a given instance of a recipe but also between that matrix and the linguistic potential that is deployed in recipes in general. The latter correlation is known as a functional variety or register of the general systemic potential.