Halliday & Matthiessen (2014: 315-6):
In the temporals there is an additional category of ‘frequency’, how many times?. This is related to the interpersonal category of usuality, but it is not identical to it; usuality is a modal assessment referring to position on a scale between positive and negative (always/never), whereas frequency is the extent of the repetition of the process. The categories of extent and usuality may, however, work together, as in a narrative where habitude is established.