Halliday & Matthiessen (2014: 29):
Looked at from the system pole of the cline of instantiation, they [text types] can be interpreted as registers. A register is a functional variety of language — the patterns of instantiation of the overall system associated with a given type of context (a situation type). These patterns of instantiation show up quantitatively as adjustments in the systemic probabilities of language; a register can be represented as a particular setting of systemic probabilities.