Halliday & Matthiessen (1999: 524):
Grammatically, each time [a speaker] says a clause, he is not only construing a process but also, unless he makes it logically dependent on another clause, acting out a speech function; and this embodies two simultaneous choices. The speaker is either giving or else requiring the other person to give — that is, demanding. And the commodity being given or demanded may be either "goods-&-services" or "information". Each of the four combinations defines one of the primary speech functions:
