Halliday & Matthiessen (1999: 509-10):
Language is set apart, however, as the prototypical semiotic system, on a variety of different grounds:
it is the only one that evolved specifically as a semiotic system;
it is the one semiotic into which all others can be “translated”; and
it is the one whereby the human species as a whole, and each individual member of that species, construes experience and constructs a social order.
In this last respect, all other semiotic systems are derivative: they have meaning potential only by reference to models of experience, and forms of social relationship, that have already been established in language. It is this that justifies us in taking language as the prototype of systems of meaning.
Beyond the narrow world of one species, Homo sapiens, language is least typical of all semiotic systems, being mostly confined to humans and being, on the SFL model, the only semiotic system with a stratified content plane.