Halliday & Matthiessen (1999: 210):
As we have seen, sensing is agnate with modality and, in the case of emotive sensing, with attitude. Similarly, qualities of projection extend to include modalities and attitudes; when the thing (or metathing) they are assigned to is agnate with the Phenomenon, these qualities are construed as objective, impersonal ones: it is certain/likely/possible that the moon's a balloon. (Contrast: I'm certain the moon's a balloon.) With qualities of usuality, this is the only possible orientation.