Halliday & Matthiessen (2004: 498):
The primary group (a) may be finite or non-finite … . The secondary group (b c d …) is always non-finite, this being the realisation of its dependent status. … The secondary group may be perfective, with or without to … or imperfective … in aspect. The other non-finite form, the ‘past/passive participle’ … usually stands for the perfective … but in itself it is neutral, and in other contexts it neutralises the distinction … .