Martin (1992: 309):
Nuclear relations reflect the ways in which actions, people, places, things and qualities configure as activities in activity sequences. The following Process Medium structures from the field of tennis illustrate the kind of relations involved. In this field, players serve aces and volley winners, but they do not *serve lobs or *volley double-faults.
serve + acesmash + overhead
put away + volley
hit + winner
net + passing shot
intercept + volley
drop + shotlob + return
volley + winner
 This continues the misconstrual of field as the semantic description of social activities. That is, context — the culture as semiotic system — is misconstrued as the language that describes social activities. See previous critique here.
 These structures are all Process + Range, not Process + Medium. (The tennis player is the Medium of each Process.) In each case, the participant refines the process by naming a particular variety of it. On Range, Halliday & Matthiessen (2004: 295) write:
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… Ordinarily, this is construed nominally, rather than clausally, as Classifier + Thing.