A use case diagram is a technique that helps to depict the interactions between a system and its actors using graphical symbols and relationships. A use case diagram shows the actors that are involved in the system, the use cases that represent the functions or services that the system provides, and the associations that indicate how the actors and use cases are related. A use case diagram can be used with a state diagram to verify that all functions are defined to work with entity “User” for the different roles, as it can help to identify the use cases that correspond to the states and transitions of the entity, and to ensure that the use cases are complete, consistent, and accurate. A sequence diagram is a technique that helps to represent the sequence and timing of the messages that are exchanged between the objects or components of a system. A sequence diagram shows the objects or components that participate in the interaction, the messages that are sent and received by the objects or components, and the time order of the messages. A sequence diagram can help to illustrate the dynamic behavior and logic of the system, but it does not show the interactions between the system and its actors. A functional diagram is a technique that helps to represent the functions or processes that are performed by a system or a solution using graphical symbols and relationships. A functional diagram shows the functions or processes that are relevant to the domain, the inputs and outputs of each function or process, and the decomposition and hierarchy of the functions or processes. A functional diagram can help to define and analyze the functional requirements, but it does not show the interactions between the system and its actors. A data flow diagram is a technique that helps to create a high-level overview of a business process by showing the movement and transformation of data between entities and processes. A data flow diagram uses graphical symbols and arrows to represent the sources and destinations of data, the processes that manipulate data, the data stores that store data, and the data flows that transfer data. A data flow diagram can help to understand and document the data logic and structure of the business process, but it does not show the interactions between the system and its actors. References: BABOK Guide v3, Chapter 10: Techniques, Section 10.50: Use Case Diagram, p. 564-566; Section 10.49: Sequence Diagram, p. 562-563; Section 10.22: Functional Decomposition, p. 528-529; Section 10.18: Data Flow Diagrams, p. 522-524.