You need a data model to write business rules. You can use an XML Schema, JDK Class or Definitions or both as the data model for implementing rules.
You can also define your own data model in the form of definitions in the process composer.
Using Schemas
Using Classes
Creating and Editing a Definition
Using Project Resources
Creating a Definition Alias
Replacing Aliases
Setting Smart Aliases
Renaming Aliases
Deleting Aliases