Halliday & Matthiessen (1999: 557):
But, thirdly, within the overall construction of experience, the diversity of spheres of social action is realised by variation in the line-up of semantic features — that is, by variation in register. The probabilities are reset; and in some cases one or two "critical systems" are strongly affected in this way, such that the local norm skews the system, or perhaps even reverses the skewing set up by the global norm. It is here that we find future taking over as the unmarked primary tense in weather forecasting. As we said above, we define register variation in just these terms, as the ongoing resetting of probabilities in the lexicogrammar, which then functions to construe the ongoing variation at the level of the social process.