You mean the schema types? If so, it's generated for the exact reason stated in the documentation:
Castor docs wrote:Some Schema types do not have a corresponding type in Java. Thus the Source Generator uses Castor implementation of these specific types (located in the types package). For instance the duration type is implemented directly in Castor. Remember that the representation of XML Schema datatypes does not try to fit the W3C XML Schema specifications exactly. The aim is to map an XML Schema type to the Java type that is the best fit to the XML Schema type.