You may need to visit QAD documentation library to get the better idea of Qxtend.
In nutshell it is interoperability framework based on SOAP architecture. It exposes QAD API's to external world or QAD application itself.
Basically there are two components of Qxtend.
1. QXO :- This is used to export the data out of QAD application in form of XML based Qdocs.
1. QXI :- This is used to import the data into of QAD application in form of XML based Qdocs.
Qdocs are integral part of QXtend and it is defined by QAD standards.