Halliday & Matthiessen (1999: 524):
The most immediate way we act, grammatically, is through our choice of speech function. One kind of speech function is a command; this is obviously a way of getting someone to do something, but we tend to think of it as being in this respect untypical. However, all speech functions are modes of action, whether command or offer, question or statement, or any other innumerable combinations or subcategories.