Halliday (2008: 66-7):
Each feature (that is, each term in each system) is entered in the network together with its realisation, the contribution it makes to the shape of the final output. In the most general (least delicate) systems, this contribution is typically structural: some change in the functional configuration, like adding new elements or putting them in a certain order. But some items of wording appear as realisations in the network almost from the start: these are the function words of English like the and of and it and and and to. As you move over towards the right, the systems become less general; and more and more of the features get to be lexicalised. First there will be middle order items, those functioning somewhere in between the grammatical and lexical poles: like pronouns, and certain types of conjunction and of adverb; then particular classes of verbs, of adjectives or of nouns.