Halliday & Matthiessen (1999: 322):
Each field is projected onto one variety of the ideation base: that is, it can be thought of as activating some portion of the total semantic resources. This projection of field onto the ideation base involves both the particular domain and the general types under which this domain is classified. A given field is thus constituted as a principled selection of types from within the ideation base; this is so to speak its semantic image projected from context.