Halliday (2008: 174, 180):
Lexical items, by contrast, form open sets; they are particular to certain domains, but open-ended and not so mutually defining: you can add a new lexical item to a set without perturbing the other members, whereas with a closed system if any term is added or taken away all the others move around. In other words, grammar and lexis are different manifestations of the principle of choice in a semiotic system. …
each [lexical item] is the intersection of a number of component features, and such features can be networked — they are systemic.