Business Rules can define a context object, whose properties are exposed and used in the expressions of the rules. For example, my context object is an object with 3 properties. PropA, PropB and PropC.
When constructing my rules, I can call a custom function, and I have one I built myself called 'myFunction' which takes arguments and I pass in properties of my context object. (I can pass in the value of the context object's PropA, B or C).