Defining Descriptive Programming

All data will either be provided directly to the system. That's the input or derived. Derived data is either immutable or immutable. The fact that such data is essential does not mean that it will correspond to essential state. So they're making this distinction between data and state. State is what has to be stored. Data is just some piece of data that may or may not have to be storage. If there is a possibility that the system may be required to refer to the data in the future, the system must retain the data. Otherwise, the data need not be maintained at all.