Halliday & Matthiessen (1999: 552-3):
We have sometimes referred to the relative frequency of a particular feature of the grammar. For instance, in our two examples of the meaning base as a resource in language processing, certain patterns characteristically recurred: future tense in the weather forecasts, imperative mood in the recipes. In each case this was a special feature pertaining to the register in question: in weather forecasts, the future tense is especially frequent relative to the other primary tenses. To say this means that there is a general expectancy in English discourse that, again relative to the other primary tenses, future will occur less frequently than it does here. In other words, there is some global expectation, in the grammar of English, about the relative frequency of the different terms in the primary tense system, past, present and future. Similarly there is some global expectation about the relative frequency of imperative and indicative mood. Frequency in the text is to be interpreted, therefore, as the manifestation of underlying probability in the system.