During unarchiving, an EOEditingContext uses the EOEditingContext class method defaultParentObjectStore to determine its parent object store. If a coordinator has not yet been created, it is created at this time. The EODatabaseDataSource verifies that its underlying EOObjectStoreCoordinator has an EOModelGroup that can service its entity. If you haven't assigned a model group to the  coordinator, then the coordinator uses the default model group-the shared EOModelGroup instance returned by the EOModelGroup class method defaultGroup . The default model group is created the first time defaultGroup is invoked.
Subsequent invocations return the same shared instance.
It contains all the models for an application, as well as for any frameworks the application references. In the majority of applications, the default model group is sufficient. This method checks to see if the application's EOObjectStoreCoordinator already has a database context that the  data source can use. If it does, it returns that database context. Otherwise it instantiates a new database context, adds it to the coordinator, and returns it.