We should not, perhaps, take it for granted that a description in terms of a formalised model, which has certain properties lacking in those derived … - Michael Halliday

Saussure took the sign as the organizing concept for linguistic structure, using it to express the conventional nature of language in the phrase "l'arbitraire du signe". This has the effect of highlighting what is, in fact, the one point of arbitrariness in the system, namely the phonological shape of words, and hence allows the non-arbitrariness of the rest to emerge with greater clarity. An example of something that is distinctly non-arbitrary is the way different kinds of meaning in language are expressed by different kinds of grammatical structure, as appears when linguistic structure is interpreted in functional terms.

[interpersonal meaning] embodies all use of language to express social and personal relations, including all forms of the speaker's intrusion into the speech situation and the speech act.

A child learning his [her] mother tongue is learning how to name; he [she] is building up a meaning potential in respect of a limited number of social functions. These functions [instrumental, regulatory, interactional, personal, heuristic, and imaginative] constitute the semiotic environment of a very small child, and may be thought of as universals of human culture