In my opinion， process instance is instance of process definition.
1 process instance admin should be different to process definition admin.
process definition admin should to browse his own process and can start this process definition.
process instance admin should to browse his own process instance and about it , can assign permissions（process instance admin） to a user.
but process instance admin may not be able to browse, or to start the process definition.
2 process instance admin can assign role (business admin) of task (in this process instance ) to other user.
and this user can do anything of business admin of task.
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