Every system has two perspectives that are reflected in the user requirements which are data and process. Any systems that exist will these two perspectives, among others, that must be addressed to build a quality application/system. In this milestone, you are creating diagrams for both the data model and process model perspectives to accurately capture the business requirements initiated in the milestones above.
Data modeling is a technique for organizing and documenting a systems data. Data modeling is sometimes called database modeling or database design because a data model is direct input into the implementation of the physical database, depending on the database platform for the organization. Data is viewed as a resource to be shared by as many processes as possible. As a result, data must be organized in a way that is flexible and adaptable to unanticipated business requirements, and that is the purpose of data modeling.
Process modeling is a technique for organizing and documenting a systems functionality. Keep in mind, data and processes/ business functions are derived from the analysis of the business requirements graphically captured in Milestone 1 and 2 deliverables above. The business functions will ultimately be translated into program code in the implementation phase of the SDLC, if we were to really build the application/system.
Notice, the process model is just another graphical approach to capturing the business functional requirements. The process model meets the same objective of creating the Use Case Diagram in the UML/object oriented analysis and design (OOAD) approach, but the symbols/syntax is slightly different given the specific objective of the two graphical approaches. You will be creating the remaining UML/OOAD diagrams to graphically capture additional requirements in the next milestone assignment.

Logical Data Flow Diagram (LDFD) (Visio)