Modulation operates on proposals, where “the meaning of the positive and negative poles is prescribing and proscribing: positive ‘do it’ and negative ‘don’t do it’ [and] there are two kinds of intermediate possibility, in this case depending on the speech function, whether command or offer” (H & M, p.147). For commands, the modality is one of obligation, which may be either subjective or objective.
Subjective obligation may have one of two orientations:
High: You must go!
Medium: You will/shall go!
Low: You may go!
The medium and low values may also then be softened by shifting to past tense: you should go; you might go.
High: I demand that you go.
Medium: I recommend that you go.
Low: I suggest that you go.
Notice that these are a grammatical metaphor shift from verbal processes, in keeping with the speech function of command.
Objective obligation may also have two orientations:
High: You’re required to go.
Medium: You’re supposed to go.
Low: You’re allowed/permitted to go.
High: It is necessary that you go.
Medium: It is recommended that you go.
Low: It is permitted for you to go.