Halliday & Matthiessen (1999: 32-3):
From a systemic-functional point of view, this means that the ideation base is construed as a multidimensional, elastic semantic space. This space is organised as a meaning potential, with an extensive system of semantic alternatives; these alternatives are ordered in delicacy. Each set of alternatives is a cline of semantic space rather than a set of discrete categories, and any alternative may be constituted structurally as a configuration of semantic rôles. The meaning potential is thus differentiated axially into (i) systems of options in meaning and (ii) structural configurations of rôles by which these options are constituted.