With the Oracle BPM Suite your process data consists of project or process variables. Whenever the variable is based on a component, that component is either defined by some external composite (like a service), or is defined by the BPM composite itself, in which case it will be a Business Object. That Business Object is created directly or is based upon an external schema. Still with me?
When using an external schema you should define the business object based upon an element instead of a complexType. Both will be possible, but when you define it based upon a complexType, you will find that any variable using it, cannot be used in (XSLT) transformations nor can be used as input to Business Rules.
As an example, see the following schema:
The customer variable (that is based on an element) can be used in an XSLT transformation, whereas the order variable cannot:
For a similar reason, the customer variable can be used as input to a Business Rule but the order variable cannot: