Halliday & Matthiessen (1999: 32-3):
Thus any account of the ideation base has to be metalinguistically stratified It has to be constructed as a theoretical model out of the resources the theory provides and according to the constraints imposed by these resources. 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 in semantic space rather than a set of discrete categories, and any alternative may be constituted structurally as a configuration of semantic roles. The meaning potential is thus differentiated axially into (i) systems of options in meaning and (ii) structural configurations of roles by which these options are constituted.
The meaning potential itself is one pole on the dimension of instantiation: it is instantiated in the unfolding of text, with patterns of typical instantiation (specific domains of meaning) lying somewhere in between the potential and the instance. At the same time, this overall ideation base can be expanded by various semogenic strategies, among which we are foregrounding that of grammatical metaphor.