Halliday & Matthiessen (2014: 293-4):
In contrast the Scope of a ‘material’ clause is not in any way affected by the performance of the process. Rather it either (i) construes the domain over which the process takes place … or (ii) construes the process itself, either in general or specific terms … There is not, in fact, a sharp line between these two; they really lie along a single continuum.