For example, a use case diagram is an expression of the relationship between roles and use cases. The traditional role system has users and administrators. User's use cases include querying broadband usage, renewing fees, modifying information, etc. The administrator's use cases are basically functions that he can operate. Analyze according to this idea.
Activity diagram is a flow chart, such as the flow chart of a user's fee renewal case. You must log in to the system, check his broadband situation, enter renewal fee, payment, etc. , and according to the normal operation order.
The following figure is an example of an activity diagram, in which the swimming lane is very important. You can refer to the trufun help system.