Two security models can be used for hierarchies, the Manager hierarchy and the Position hierarchy. The Position hierarchy allows data access across business units. If you are a financial organization, you may prefer the Manager hierarchy model, to prevent managers' accessing data outside of their business units. However, if you are a part of a customer service organization and want the managers to access service cases handled in different business units, the Position hierarchy may work better for you.
Note: The hierarchy security model is an extension to the existing security models that use business units, security roles, sharing, and teams. It can be used in conjunction with all other existing security models. The hierarchy security offers a more granular access to records for an organization and helps to bring the maintenance costs down.
Reference:
https://docs.microsoft.com/en-us/power-platform/admin/hierarchy-security