Halliday & Matthiessen (2014: 151-2):
(i) It specifies time reference other than reference to the time of the speech event, that is, ‘secondary’ tense: past, present or future relative to the primary tense.
(ii) It specifies various other aspects and phases such as seeming, trying, hoping.
(iii) It specifies the voice: active or passive.
(iv) It specifies the process (action, event, mental process, relation) that is predicated of the Subject.