Halliday & Matthiessen (2014: 692):
The basic distinction that determines how each type of modality will be realised is the ORIENTATION: that is, the distinction between subjective and objective modality, and between the explicit and implicit variants… . The system is as in Figure 10-7.
These combine with all four types of modality, but with gaps; for example, there are no systematic forms for making the subjective orientation explicit in the case of usuality or inclination (i.e. no coded expressions for ‘I recognise it as usual that ... ’ or ‘I undertake for ... to ... ’). This is a systematic gap; these particular combinations would represent semantic domains where the speaker cannot readily pose as an authority.