Halliday & Matthiessen (2014: 180-1):
This paradigm shows that probability is organised as a system of three values: a median value ‘probable’ where the form of the negative is the same whether it is attached to the modality or the proposition, and two outer values, high ‘certain’ and low ‘possible’, where there is a switch from high to low, or from low to high, if the negative is shifted between the two domains.
All nine feature combinations may be realised by Finite operator, modal Adjunct, or both. Exactly the same set of possibilities arises in respect of the three other dimensions of modality. …
It is this parallelism in their construction of semantic space, all lying within the region between the two poles of positive and negative, that gives the essential unity to this particular region of the grammar.