Halliday & Matthiessen (1999: 122-3):
Texts and sequences are of the same order of abstraction; both are semantic phenomena. A text is a piece of language that is functional in context. It draws on the ideational meaning base but it involves the full metafunctional spectrum; i.e. there are interpersonal and textual contributions as well. Since text draws on the ideational meaning base, sequences are one principle for organising text.