Halliday & Matthiessen (1999: 122):
Sequences impose a certain order on our experience in terms of the relations that connect [one] happening with another. Hence sequences can be used to store information about the world in the form of organised text — ‘this is how to change tyres on your car’, ‘this is how to make cauliflower surprise’, etc.. Such texts often fall into a clearly recognisable text type, such as procedures, proofs, explanations, and episodic narratives. Not all texts are as highly regulated as these; but it is usually possible to make some prediction about the kinds of sequence, and the complexity to which sequences extend, in most our culturally recognised modes of discourse.