Modalization operates on propositions, in the space between ‘It is…‘ and ‘It isn’t…‘. There are “two kinds of intermediate possibilities: (i) degrees of probability [and] (ii) degrees of usuality” (H & M, p.147). Usuality is “equivalent to ‘both yes and no’, that is, sometimes yes, sometimes no, with different degrees of oftenness attached”. Here is a description of Koala feeding patterns from Australian Wildlife:
They will feed at any time of day, but usually at night.
This demonstrates the two types of orientation for the system of usuality.
1. Subjective (implicit)
High: It (does) rain.
Medium: It will rain.
Low: It may rain.
2. Objective (implicit)
High: It always rains.
Medium: It usually rains.
Low: It sometimes rains.
Unlike probability, there is “no systematic form for making the subjective orientation explicit” (H & M, p.619) but it is possible to make the objective orientation explicit with items such as, It’s common for it to rain; It’s usual for it to rain.