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Knowing that users belong to departments and that departments have their inherent hierarchical structure makes it understood how big a part the Head-of-Department (HOD) role plays in the setup of your chain of reporting. Each department can be assigned one HOD and the HOD will play a prominent role in process participants assignments.
To assign a user to the HOD role, you can either access the specific user account and select HOD as true, or you can access the department page (Setup Users > Setup Organization Chart > click on an organization > click on a department) and create your assignment there.
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