Halliday & Matthiessen (1999: 3-4):
A systemic grammar is one of the 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 a formal, lexicogrammatical reactance. [The reactance of a category is its distinctive treatment.]
Looked at from the formal angle, of course, this means that it is likely to appear complex; many of the categories are "cryptotypic", manifested only through a long chain of realisations (a "realisational chain"). Hence it takes a long time and a great deal of effort to get such a grammar off the ground in any context (such as natural language processing) requiring total explicitness. Once airborne, however, because it is semantically natural it has considerable potential as the basis on which to represent higher level organisation — provided, that is, such organisation is interpreted in linguistic terms, as meaning rather than as knowledge.