Martin (1992: 317-8):
At clause rank, the TRANSITIVITY roles which have not been considered are Agent and Beneficiary (including Client, Recipient and Receiver). These roles are intermediate between extensions and enhancements, and are accordingly realised both with and without prepositions in clause structure:
Agent: +preposition The ship was stolen by Ford.
–preposition Ford stole the ship.
Client: +preposition He bought the present for Trillian.
–preposition He bought Trillian the present.
Recipient: +preposition He gave the drink to Zaphod.
–preposition He gave Zaphod the drink.
Receiver: +preposition He told the story to Arthur.
–preposition He told Arthur the story.
In line with the proposals developed above, the prepositional realisations of these intermediate roles can be taken as enhancing and the non-prepositional realisations as extending.
 No argument is provided as to why Agent and Beneficiary rôles are "intermediate between extensions and enhancements" — it is merely asserted as a fact. In SFL theory, the expansion relation between the Nucleus and both the Agent and the Beneficiary is enhancement, not extension (Halliday & Matthiessen 1999: 175, 219), not least because
- the participant Agent (external cause) is agnate with the circumstance Manner: means;
- the participant Client is agnate with the circumstance Cause: behalf;
- the participants Recipient and Receiver are agnate with the circumstance Location: directional.
 The reason for the variation in participant realisation — as prepositional phrases or nominal groups — has nothing whatsoever to do with expansion type. Halliday & Matthiessen (2004: 295-6):
… the choice of ‘plus or minus preposition’ with Agent, Beneficiary and Range … serves a textual function. … The principle is as follows. If a participant other than the Medium is in a place of prominence in the message, it tends to take a preposition (i.e. to be construed as ‘indirect’ participant); otherwise it does not. Prominence in the message means functioning either
(i) as marked Theme (i.e. Theme but not Subject) or
(ii) as ‘late news’ — that is, occurring after some other participant, or circumstance, that already follows the Process.
In other words, prominence comes from occurring either earlier or later than expected in the clause; and it is this that is being reinforced by the presence of the preposition. The preposition has become a signal of special status in the message.Again, function is here being classified according to form (the view 'from below' of formal linguistics) instead of form being classified according to function (the view 'from above' of functional linguistics).