Halliday & Matthiessen (2014: 260):
Unlike ‘material’ clauses, but like ‘mental’ ones, ‘relational’ clauses prototypically construe change as unfolding ‘inertly’, without any input of energy — typically as a uniform flow without distinct phases of unfolding (unlike the contrast in material processes between the initial phase and the final phase of the unfolding of a process, the outcome).