Sequence Diagram vs Usecase Diagram

A use case diagram shows how use cases and actors relate to each other and gives a functional system overview.

A use case is graphically represented in an activity diagram where the single steps occur as actions.

A sequence diagram is used to show how objects communicate.

Usually you derive classes and relate them to actions of the use case activities. So when you create a sequence diagram it highlights a certain aspect of the whole system. This highlight is usually spot on and not a floodlight. Putting multiple use case scenarios in a single SD would for sure simply blind any reader instantly.

Usecase diagram

Use case diagrams show business use cases, actors, and the relationships between them. The relationships between actors and business use cases state that an actor can use a certain functionality of the business system. You will not find any information about how or in what chronological sequence these services are rendered

sequence diagram

A sequence diagram can map a scenarios described by a use case in step by step detail to define how objects collaborate to achieve your application's goals.

Here is the diagram , which will clear things better to you