It is not always clear when to use a use case and when not to use a use case. If a use case is used, how and what should be described in the use case?

A use case can only be used to describe a specific usage of the system. The use case clearly describes the interaction of an actor with the system: it is a sequence of 'the actor does this, the system does that'. An actor can be human (user) or non-human (hardware device). It is important that the actor can only be something that is outside of the system boundaries: it can't be a subsystem of the system itself.

The description of the flow of a use case should be limited to describing the interaction of the actor with the system. It should not include lengthy specifications related to a specific use case step. For example, when the use case flow states 'the system shows order information', the specification of the order information is best done outside of the flow.

If this non-functional specification is related only to one use case, it can be added in the section of the use case document describing the non-functional specifications, for example under a heading 'Order Information'. If the specification is relevant to more than one use case it can be added as a requirement of the system, for example 'The system must provide order information'. The detailed specification of the requirement then describes the order information in detail.