Halliday & Matthiessen (2014: 494-5):
The elaborating clause is either (a) finite, where the relative is who(m), which, that or implicit (a ‘contact’ relative), or (b) non-finite, where the relative is typically implicit; …With the non-finite clauses, note again the difference between imperfective and perfective, as in the following set:
[imperfective:](a) operative the person taking pictures (‘who is/was taking’)(b) receptive the pictures taken by Mary (‘which were/are taken’) (according to the tense of the outer clause)
[perfective:](a) operative(1) the (best) person to take pictures (‘who ought to take’) [relative = Subject]
(2) the (best) pictures to take (‘which someone ought to take’) [relative = Complement](b) receptive the pictures to be taken (‘which are/were to be taken’)
Glosses in parenthesis suggest the nearest equivalent finite form. In non-finite elaborating clauses, the implicit relative is normally the Subject, but in perfective operative clauses it may be either the Subject (as in the person to take pictures) or the Complement (as in the pictures to be taken). Here we thus see two principles in operation, viz. (1) the Subject may be presupposed in a non-finite clause; (2) the Complement may be presupposed in a defining relative clause. The second principle also extends to Adjuncts, as in the best time to take pictures (‘the best time at which to take pictures’); these are treated as enhancing.