Halliday & Matthiessen (1999: 204-5):
Things of the enhancing type […] construe one or other of the general logical semantic relations of cause, manner, time and place. As Facet, they specify some circumstance of an element functioning as Thing: the cause/result/purpose of the breakdown, the time/occasion of the breakdown, the place/location of the breakdown; cf. also the circumstances of the breakdown. As Thing, they give the logico-semantic relation itself the status of participant; here the usual words are reason, way, time, place, and the phenomenon becomes a figure realised by a qualifying clause: the reason/way/time/place we broke down. These are closest to the borderline with projections: we may have either expansion the reason for which we broke down, or projection (the reason) why we broke down. Those with ‘reason’ and ‘way’ often enter into an identifying relation with some other figure; for example, the reason we broke down was that/because the engine overheated.