In runtime the mapping metadata is accessed via a DataChannel in a form of org.objectstyle.cayenne.map.EntityResolver (EntityResolver available via ObjectContext is usually obtained from the channel behind the scenes). EntityResolver provides a single namespace for multiple DataMaps.
Another important function of the EntityResolver is providing access to the org.objectstyle.cayenne.property.ClassDescriptor instances for each persistent entity. Those a compiled by Cayenne on the fly using EntityResolver's org.objectstyle.cayenne.property.ClassDescriptorFactory.