Halliday & Matthiessen (2014: 177):
In a proposition, the meaning of the positive and negative pole is asserting and denying; positive ‘it is so’, negative ‘it isn’t so’. There are two kinds of intermediate possibilities: (i) degrees of probability: ‘possibly/probably/certainly’; (ii) degrees of usuality: ‘sometimes/usually/always’.
The former are equivalent to ‘either yes or no’, i.e. maybe yes, maybe no, with different degrees of likelihood attached. The latter are equivalent to ‘both yes and no’, i.e. sometimes yes, sometimes no, with different degrees of oftenness attached. It is these scales of probability and usuality to which the term ‘modality’ strictly belongs. We shall refer to these, to keep them distinct, as modalisation.