This guide will explain the differences between Approval by Department & Approval by Hierarchy.
Approval by Department: Approval is based on the department where the applicant belonged. Each department can be approved by different users. If there is more than one approver who is assigned to a department, leaving application will need to be approved by all these approvers.
Approval by Hierarchy: Approval is based on the hierarchy of superiority. If Employee A is the superior of Employee B, then Employee A will be the approver. If there is more than one approver, there is an option to define who is(are) required to approve a leave application based on the level of hierarchy. As abbreviation, the decisions are made at different levels of authority within an organization.