Probably the best known diagram in UML are the Use Case Diagrams. Many projects begin with a set of use cases that show what the system should be able to do, in terms of functionality, once implemented. The Use Case Diagram is a simple, yet useful, way of showing these use cases diagrammatically.
It is of vital importance that you take immense care when designing your use cases. The use cases that you design now will be used as the basis each and every diagram to follow! No, I am not exaggerating. Every diagram depends on these use cases! Each one! Well, except for the Entity Relationship Diagram, but in a way as well. So, yes! Every diagram! Okay, except Component and Deployment Diagrams. But, every other diagram! Right…
The four main components in Use Case Diagrams are:
Use Case Diagrams only show functionality that will be present in the system and not how this functionality will be implemented.
Use Case Diagrams are only a set of actions, services, and functions. Do not show processes and dataflow in your diagram.