I can suggest two ways to learn about it:
(1) Either you study the BPMN 2.0 specification. It contains
CMOF files that define the official BPMN 2.0 meta-model. It goes far beyond the XSD schema people use to validate the structural correctness of BPMN 2.0 XML documents.
(2) Inspect bpmn-moddle and the meta-model descriptors we generate from the offical
CMOF descriptor files.
You’re probably better of to look into our descriptors first, as we provide numerous fixes to the official meta-object descriptors. These fixes adapt the model to allow proper exporting to and importing from valid BPMN 2.0 XML. For instance we correct the order of elements to match the structure described in the BPMN 2.0 XSD schema.
Hope this helps!