Halliday & Matthiessen (2014: 573):
Again, there are two dimensions: there is the potential, and the actual. The potential means having, or alternatively not having, the ability to succeed. The actual means trying, or not trying; and succeeding, or not succeeding. The form with have has evolved like the forms with be above. Originally two verbal groups, it is now either (i) + done, a secondary tense form ‘past in’, e.g. has done ‘past in present’, will have done ‘past in future’, was going to have done ‘past in future in past’ and so on; or (ii) + to do, a modal form (of the ‘modulation’ type), e.g. has to do ‘must do’. In other words, ‘possessing’ a process, if combined with past/passive, means past (success); if combined with ‘unreal’, it means (future) obligation.