Halliday & Matthiessen (1999: 378-9):
Semantic features are realised by lexicogrammatical ones; we have illustrated this relationship within the ideational metafunction at various points in our discussion (e.g. sequence realised by clause complex; figure of doing realised by material clause). The realisational relationship between semantics and lexicogrammar is one of preselection: semantic features such as 'sequence', 'figure', and 'doing', are realised in lexicogrammar by means of prespecification of lexicogrammatical information, most centrally preselection of lexicogrammatical features.
For instance, 'doing' is realised by the preselection of the clause feature 'material', which means that the clause that realises a figure of doing is constrained to be a material clause.