Martin (1992: 93-4):
But reference of this kind is not really appropriate in written mode, where context independency is functional across most contexts.
This confuses semiotic context with material setting. In SFL theory, there is no "context independency" and so it is not "functional across most contexts", even in written mode. The stratal relation between language and context is one of symbolic abstraction, such that language realises context. In terms of semogenesis, language construes (intellectually constructs) its cultural context; the cultural context is construed by means of language.