Halliday & Matthiessen (1999: 606):
These systems enter into relation with language in two ways.
On the one hand, they are metonymic to language: they are complementary, non-linguistic resources whereby higher-level systems may be realised (e.g. ideological formations realised through forms of art; theoretical constructs realised through figures and diagrams).
On the other hand, they relate metaphorically to language: they are constructed, stratally and metafunctionally, in the image of language itself, and hence can be modelled on language as prototype, being described "as if" they had their own grammar and semantics …
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Importantly, in SFL terms, only language has a grammar; only linguistic texts can be read out loud (rather than merely described).