Halliday & Matthiessen (2014: 56-7):
This introduction to (systemic) functional grammar differs in various ways from other accounts — in terms of both theory and description. … (ii) In terms of description, this book is of course an introduction to a systemic functional description of the grammar of English — constituting one descriptive strand evolving among other ones in systemic functional linguistics. This description may be compared with other descriptions of the grammar of English that have appeared over the past 500 years or so. These descriptions naturally vary in many ways, e.g.
- relationship to theory (homogenous or heterogeneous [‘eclectic’]),
- relationship to corpus, relationship to time (diachronic vs. synchronic, or some kind of synthesis),
- relationship to dialectal variation (what varieties of English are included),
- coverage of phenomena — from grammars of very selective coverage via grammars with a registerial focus (such as grammars of spoken English) to reference grammars, and
- relationship to intended users — ranging from language learners to professional grammarians.
Reference grammars are, in principle, the most comprehensive descriptions.