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If you don't specify anything here, the transition on the related issue will be performed with the same user who triggered this post function on the origin issue. ThusTherefore, that user must have the necessary permissions on the related issue.
However, in In some restrictive setups, that a user might not be allowed to do so on the related issue. He might not even see the project of the related issue!perform the action or might not have permission to view the relevant project.
With 'Perform As User', you can specify a different user account that has been granted the necessary permissions. Usually, this user account is assumed to be only technical (impersonation), with broad permissions, and not used to log into Jira by any coworkers.
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