Halliday & Matthiessen (1999: 122):
Sequences impose a certain order on our experience in terms of the relations that connect 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 of our culturally recognised modes of discourse.
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In SFL Theory, text type is register viewed from the instance pole of the cline of instantiation. Text types can be identified according to the MODE that they realise (e.g. procedure, proof, explanation, episodic narrative) — MODE being the textual dimension of context: the rôle of language in a situation.