At this point it may still not be completely clear how we can decide in a particular model if some data should be modeled as Processing Data or a Control. There are several issues though that can illustrate the existing difference between Processing Data Objects and Controls.
To start with, Controls are simple data types such as integers, floats or boolean. Any structure more complex than that cannot be sent as a Control. This feature may be seen as a limitation of the metamodel but it is in fact a feature. It is important to bear in mind that controls are sent asynchronously whenever they generate or are modified. The control associated to a slider, for instance, can be transmitted several times before actually affecting the result of the process, which will happen next time that the Processing Do is called. It would be a waste of resources to transmit complex structures with this mechanism.