How is the passivation of Entity beans Managed?
There are two ways for transitioning an entity bean from the ready to the pooled state, by using the ejbPassivate() or ejbRemove() method. The container uses ejbPassivate() to disassociate the bean instance from the entity object identity, and uses ejbRemove() to remove the entity object.
When the instance is put back into the pool, it is no longer associated with an entity object identity. The container can now assign the instance to any entity object within the same entity bean home.
A more thorough discussion can be found in the EJB 1.1 specification by Sun.