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Phase 1 of the UMA core protocol involves the resource owner introducing the resource server and authorization server so they can work together. Phases 2 and 3 together involve the requesting party, using a client, making an access attempt, being tested for suitability by the authorization server to receive permission, and ultimately succeeding or failing in the attempt by presenting a token with permissions associated with it.
Further reading:
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UMA's Relationship to Other Efforts
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