Halliday & Matthiessen (2014: 292):
In this type, it is the participants — Identified and Identifier — that are circumstantial elements of time, place and so on. For example, in tomorrow is the tenth, tomorrow and the tenth are both time elements. Similarly in the best way to get there is by train, both the best way and by train express manner; in the real reason is that you’re scared, Identified and Identifier are both expressions of cause. The Token can be quite varied in grammatical class — a nominal group, an adverbial group, a prepositional phrase or an embedded clause, whereas the Value is often a nominal group with the name of a class of circumstance as Thing.