今天学习了XPDL的一些知识，总结一下：
The Concept of the Process Definition Interchange
Meta-Model Framework
basically, it's a set of entities, such as Workflow Process Definition, Activity Set, Block Activity, Workflow Relevant Data, Workflow Participant, Resource Repository&Orgnizational Model, Transition Information, etc.
Package:
One can consider it as a container, information about the process and resource data can be included, in order to be shared by activities defined inside.
Dummy Activity:
There is a task defination in body of the activity, and you can specify the body as NUll, this is so called Dummy activity. When it's hard to present the transition between activities for complex business logics, you can use Dummy Activity.
