Auditable Units

What is the concept behind Auditable Units in Audit Management? I am trying to create one, and when I try to add records under the 'Business Units,' 'Department', etc., the records available are the actual records from Business Unit and Department tables. I was expecting the entities for type business units or departments.

Why are we adding actual records instead of entities into the Auditable Unit?

Does the risk score populated at the assessment roll up to the entity?

What happens after the assessment?

Thanks