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A scope is used to convey what permissions your application requires when requesting permission from the end-user. Once an access token is issued you cannot change the scope without recreating another grant request because the end-user has only permitted your application access within the bounds of the scope. You should see the scope as an agreement between your application and the end-user. The scope is presented to the end-user on grant request, and only if they agree to provide access will you received receive the credentials required to subsequently acquire access.
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