Business Data Objects (BDO) are containers for describing a business concept. Business Data Objects allow you to group entities into groups. You may wish to use Business Data Objects as a way to describe entities/tables that share a common relationship.
A business concept such as a customer, sales order, or order item, is often fully described by multiple entities/tables that share a common relationship to a central (root) entity table. Therefore, for you to better describe and reuse your core business concepts, you can use Business Data Objects to contain the necessary entities/tables in a single object that can be easily displayed within a model, reused across submodels and, in the case of a physical model, generate DDL from.
The entities within a BDO can have relationships to entities in other BDOs and also to entities that are not part of a BDO.
To know more check out this video on BDOs
To learn more check ER/Studio Enterprise Team here