Halliday & Matthiessen (2014: 347):
There may be in each type of clause one element which is not so much an entity participating in the process as a refinement of the process itself. This may be the name of a particular variety of the process, which being a noun can then be modified for quantity and for quality … Since here the kind of action, event, behaviour, sensing or saying is specified by the noun, as a participant function, the verb may be entirely general in meaning … Or, secondly, this element may be an entity, but one that plays a part in the process not by acting, or being acted upon, but by marking its domain … It is characteristic of this second type that they are on the borderline of participants and circumstances; there is often a closely related form of prepositional phrase …