JSU Add-On User (default): The default setting for Perform as User is JSU Add-On User. If you don't specify a different option, the transition on the related issue is performed as the JSU app user with its associated permissions.
Initiating User: The transition on the related issue is performed with the same user who triggered this post function on the origin issue. That user must have the necessary permissions on the related issue. In some restrictive setups, a user might not be allowed to perform the action or might not have permission to view the relevant project.
Perform As User: Use this option to 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.
In combination with the Permission Condition in native Jira, or the User Is In Any Users Condition from JSU, you can hide a transition from all users that do not have permission to execute it.