Use case diagrams are abstract high-level view of functionality. They do not describe use cases interactions. According to the UML specification, the realization of a use case may be specified by a set of collaborations that define how instances in the system interact to perform the sequences of the use case. The collaborations may be captured using a variety of notations including natural language, sequence diagrams, activity diagrams and state diagrams.
In practice in order to allow for an easy communication with stakeholders, use cases collaborations are usually written as structured natural language interactions between actors and a system. Different templates and guidelines for use cases edition have been proposed in the literature. UCEd template is largely based on Cockburn's where use cases are described using structured text.
We distinguish two types of use case descriptions corresponding to the two types of use cases. Normal use case description for normal use cases and extension use case description for extension use cases.