Halliday & Matthiessen (1999: 3-4):
A systemic grammar is one of a class of functional grammars, which means (among other things) that it is semantically motivated, or “natural”. In contradistinction to formal grammars, which are autonomous, and therefore semantically arbitrary, in a systemic grammar every category (and “category” is used here in the general sense of an organising theoretical concept, not in the narrower sense of ‘class’ as in formal grammars) is based on meaning: it has a semantic, as well as formal, lexicogrammatical reactance. [The reactance of a category is its distinctive treatment.]