Halliday & Matthiessen (1999: 563):
In addition, the ideation base is polysystemic in another sense: registerial variation. We have seen that such variation can be construed in terms of the probabilistic nature of the linguistic system, as variation in the probabilities associated with terms in systems. Seen in this light, a register is a particular probabilistic setting of the system; and the move from one register to another is a re-setting of these probabilities. What is globally the ‘same’ ideational semantic system can thus appear as a collection of different systems, as one [i.e. our viewpoint not the speaker] moves along the cline of instantiation from potential to instance… . As we noted above, the effect is quantitative; but it is also qualitative, in the sense that it provides different perspectives on experience within the same system.