When a modal verb is used to affect a situation, by giving permission, etc, this is deontic modality:
You can go when you've finished.
Here, the speaker is giving permission, so there is deontic modality used to control the situation.
See also: Auxiliary Verb; Dynamic Modality; Ditransitive Verb; Dynamic Verb; Epistemic Modality; Finite Verb; Inchoative Verb; Intransitive Verb; Irregular Verb; Non-finite Verb; Phrasal Verb; Regular Verb; Stative Verb; Transitive Verb; Verb Group; Verb Phrase
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