Halliday & Matthiessen (1999: 375):
… stratal organisation means that it is crucial to specify the realisational relations between strata — inter-stratal realisation. In systemic theory, this relationship is stated in terms of the organisation of the higher stratum — for a simple reason: a higher stratum provides a more comprehensive environment than a lower one (as our strata! figures withconcentric[co-tangential] circles suggest). For instance, a certain semantic feature such as 'probability' may be dispersed in realisation throughout various regions of the grammar (I think; in my opinion; probably; will).More specifically, inter-stratal realisation is specified by means of inter-stratal preselection: contextual features are realised by preselection within the semantic system, semantic features are realised by preselection within the lexicogrammatical system, and lexicogrammatical features are realised by preselection within the phonological/graphological system. This type of preselection may take different forms between different stratal boundaries, but the principle is quite general.