Halliday & Matthiessen (1999: 14):
The ideation base has one other primary dimension, that of instantiation. The ideation base is not just a repository of particular facts and other instantial meanings but also a systemic network of meaning potential. For any given domain the ideation base incorporates not only the known particulars of that domain but also the resources necessary for assimilating new information. Instantiation refers to the move from the semantic potential within the general system to instances of this potential within a particular text (cf. Halliday, 1973, 1977, 1992). Intermediate between these two on the instantiation cline are patterns of instantiation that recur in particular situation types — semantic domains located within the overall meaning potential as situated variants of it.