Validation is the process of verifying that an XML document meets all the constraints expressed in the schema. JAXB 1.0 provided validation at unmarshal time and also enabled on-demand validation on a JAXB content tree. JAXB 2.0 only allows validation at unmarshal and marshal time. A web service processing model is to be lax in reading in data and strict on writing it out. To meet that model, validation was added to marshal time so one could confirm that they did not invalidate the XML document when modifying the document in JAXB form.