Taken from this discussion thread
The DDMS Extensible Layer (pre-DDMS 5.0) allows arbitrary content at the end of a DDMS metacard. If this extensible layer uses type substitution (via xsi:type), the underlying XMLReader will fail validation, as the schema from which the type originates is not in the external schema chain. Need to investigate further to decide course of action. Likely outcomes (mutually exclusive):
1) Enforce only syntax correctness in the Extensible Layer, not external schema content.
2) Allow the XMLReader to discover referenced external schemas that allow the Extensible Layer to be fully validated when it contains type substitution.
3) Provide configuration options for one or more of the above.