Halliday & Matthiessen (2014: 172):
POLARITY [is] the opposition between positive (It is. Do that!) and negative (It isn’t. Don’t do that!); MODALITY [is] the speaker’s judgement, or request of the judgement of the listener, on the status of what is being said (It could be. Couldn’t it be? You mustn’t do that. Must you do that?). Both POLARITY and MODALITY are realised through the Mood element, either through the Finite element (It is/It isn’t; It is/It must be) or through a separate mood Adjunct (It is/It is not; It is/It certainly is).