Halliday & Matthiessen (2014: 243):
Scope, Recipient and Client are clearly treated by the grammar as participants; for example, they are all candidates for subjecthood in a ‘receptive’ clause. However, at the same time, they are clearly located some distance towards circumstances on the cline between participants and circumstances, which is reflected in the fact that that, under certain conditions, they may be marked by a preposition [minor Process].