z/Server uses the XML system parser functionality (that can run on specialty engines) for parsing XML formatted documents. Any return and reason codes displayed were originally issued by the XML system parser. For details, see SA23-1350-xx: z/OS XML System Services User's Guide and Reference.
XML is also used for communication inside z/Server. This means that XML errors may be produced in unexpected situations. The logon parameters (user ID and password) are passed as an XML document to the validation routine, and a user ID that is too long (more than eight characters) results in an XML0003E error message.