In the documentation: http://wiki.ds.unipi.gr/display/TOOP/TOOP+Exchange+Data+Model+Specification#TOOPExchangeDataModelSpecification-DataModel:TOOPRequest

It says a request (and corresponding response) can have either the DataElementRequest OR the DocumentRequest. In the xsd, however, both elements are present in a sequence, and both are allowed.

Suggest to either change the documentation (this is the preferred option), or add a xs:choice for these two elements, ensuring that one, and only one is present.