Currently there is some documentation on Item Definitions
http://stage.docs.camunda.org/manual/develop/user-guide/dmn-engine/item-definition/#item-definition-of-input-clause

We should review and refactor this in the following way:

• in the userguide there should be a section on "How to work with Data"
• in the dmn reference there should be considerations like how ItemDefinitions work etc.

Short discussion required.