What kind of entity bean are we talking about here? The responsibility of creating the actual entity in the data store is not the same for BMP and CMP.
In CMP, after ejbCreate(), the container creates the entity representation in the database. You just set the PK values and return null from ejbCreate(). --> The SCBCD exam focuses on CMP
In BMP, it is the responsibility of the bean provider for creating the entity representation in the database. You return the primary key from ejbCreate() to the container so that the container can identify which data your instance represents.