Language... is a range of possibilities, and an open end set of options in behavior that are available to the individual in his existence as social m… - Michael Halliday

[... language, as an example of a semiotic system, contains all four levels of organization as follows:] First, it is transmitted physically, by sound waves traveling through the air; secondly, it is produced and received biologically, by the human brain and its associated organs of speech and hearing; thirdly it is exchanged socially, in contexts set up and defined by the social structure; and fourthly it is organized semiotically as a system of meanings

The theme is what is being talked about, the point of departure for the clause as message, and the speaker has within certain limits the option of selecting any element in the clause as thematic.

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