Impact of using Busniess Group rather than Department to create Groups

Summary:

Hello,

We are currently implementing Core HR for a client and would like to know the impact of using Business Unit rather than Department to store their Groups.

Problem Description:
The client has a hierarchical structure of Department in 3 levels: Group --> Department --> Sub-Department.
We proposed the client to define the Groups, Departments and Sub-Departments under the Department object. However, their concern is that they want to see the hierarchical structure of these departments displayed when creating a new Position.
Since the Position screen has a field to input Business Unit, they wanted to use this field to include their Groups.