Halliday & Matthiessen (1999: 385-6):
The strata are ordered in symbolic abstraction, but they are not ordered in instantiation time. The process of instantiation can shunt up and down the stratal hierarchy. However, the general tendency in instantiation is one of stratal descent. First systemic features are instantiated (selected) at the highest stratum and their associated realisation statements are also instantiated (executed). Then the instantial specifications at this stratum are realised at the stratum below.
Within this overall stratal descent, there is interleaving: higher-stratal systems need not be fully instantiated until lower-stratal ones have been instantiated. This means that selections at higher strata can be made in the logogenetic environment of preceding selections at lower strata. This reflects the general theoretical principle that the relationship between two strata is a solidary one, with instantiation proceeding “dialogically”.