Halliday & Matthiessen (2014: 312):
We can make a contrast, then, between direct and indirect participants, using ‘indirect participant’ to refer to the status of a nominal group that is inside a prepositional phrase. We have already seen that the participant rôles of (1) Client, Recipient and Receiver [i.e. Beneficiary] and (2) Scope, Behaviour and Verbiage [i.e. Range] are sometimes expressed ‘indirectly’ in this sense, as in gave money to the cashier, plays beautifully on the piano. The elements we are treating as ‘circumstantial’ are those in which the participant typically — and in many cases obligatorily — is indirect, being linked into the process via some preposition or other.