Managed Object Model Bundles
With Leopard, there is a new type of managed object model available for Core Data–based applications. Instead of having a single managed object model file, you have a bundle of managed object models. This bundle contains all the possible managed object models that your application has used during its production lifetime.
It is this bundle that Core Data looks at to find the appropriate model for the data store that is being loaded from disk.
Creating a managed object model bundle is very straightforward. You simply select the current managed object model you are working with and from the main menu select Design -> Data Model -> Add Model Version. If you did not have a bundle previously, then Core Data will create one for you and add your existing model to the bundle.
This bundle is treated the same as a Managed Object Model (mom) document and can be loaded exactly the same way. (The file extension, however, is momd instead of mom.)