Halliday & Matthiessen (1999: 187):
By comparison with other elements, things tend to stand out(i) by their varied rôles as elements in figures,
(ii) by the overall weight and discursive force of their primary categorisation of experience,
(iii) by their tendency to be elaborated into numerous micro-categories,
(iv) by their complex internal organisation, and
(v) by their highly systematic relationship one with another.