The grammatical system has … a functional input and a structural output; it provides the mechanism for different functions to be combined in one utte… - Michael Halliday

[The construal of context as a semiotic construct accompanies the construal of language as a metafunctionally organized system because of the realizational relationship between the two. And it is this relationship which helps to explain how language is learned:]
It is this that enables, and disposes, the child to learn the lexicogrammar: since the system is organized along functional lines, it relates closely to what the child can see language doing as he observes it going on around him.

The interpersonal function [of language] is the function “to establish, maintain, and specify relations between members of societies”

[Register] is set of meanings, the configuration of semantic patterns, that are typically drawn upon under the specified conditions, along with the words and structures that are used in the realization of these meanings.