Controllers must be able to describe waht the data processing is intended to accomplish (i.e., the primary purposes for which the data are collected and used). This can be a high-level description of the services. This can briefly describe the service, the data subjects whose personal data is processed as well as the purpose/value derived from the processing system. Put simply, why does this exist?
You should also be able to define how processing will be divided between the controller and the processor and whether any joint processing will take place.