Commentary on Meditations: B3:9

Revere your power of judgement. All rests on this to make sure that your directing mind no longer entertains any judgement which fails to agree with the nature or the constitution of a rational being. And this state guarantees deliberate thought, affinity with other men, and obedience to the gods.

Our mindfulness and judgements (which is the discipline of assent) should align with the nature of the whole, in that we should love our fate (which is the discipline of desire).

Deliberate thought (which is the discipline of assent); good social interaction (which is affinity with others) and loving what happens to us (which is fate and belongs in the category of things not up to us).