Halliday & Matthiessen (1999: 554):
This illustrates one very powerful feature of a probabilistic system of this kind: that it accommodates systematic functional variation. We have discussed the two text types, weather forecasting and recipes, as examples of variation in register: of the way in which the meaning selections in texts tend to vary systematically with their contextual function — their value in the social process. We pointed out that this variation is seldom categorical: except in very closed registers, or "restricted languages", no major options are likely to be totally excluded. What happens is that the probabilities are reset. This may be a relatively minor skewing affecting a large number of semantic features; but one or other system may stand out by being particularly clearly realigned, as happens with tense and with mood, respectively, in our two examples. We can in fact define register variation as the resetting of probabilities in the lexicogrammatical and semantic systems, including those in the ideation base. We observe these probabilities in the form of frequencies in the text;