Hi,
see for example the Wikipedia article about QNames. They cannot start with a digit.
For all the places where QNames are used you can have a look at the XSDs of BPMN 2.0 at the OMG. Especially the Semantic.xsd which contains most of the non DI related BPMN elements. QNames are used whenever another element is references by its namespace and id.
Cheers,
Sebastian