The relationship between use cases

Three kinds of relationships between use cases :
(1) extension (extends): use case B extends use case A, which indicates that use case b is an extension use case that may occur in a particular case. For example: Lao Wang in the city, 2 hours can go back, in this 2 hours bursting will go to the toilet. The toilet use case is an extension of the use case for the city, as the old King does not go to the toilet to do business.
(2) contains (includes): use case a includes use case B, which means that the use case a itself is not complete without use case B. For example: or Old Wang in the city, he came to Beijing from Hainan office, 3 days to go back, then this situation in the city of the use case and toilet use case relationship should be included in the relationship .
(3) Generalization: The generalization relation refers to the different technical realization of the same business purpose. For example: Old Wang went to town, he could fly, he could take a train, he could walk, then the use case of the city was generalized to three use cases by plane, train and walk, and there was a hierarchical relationship between them.
In general, extensions can "freeze" the basic use cases to remain stable (because the extension use cases are often indeterminate) and contain different technical implementations that can provide common interactions, improve "reuse", and generalize for the same business purpose. There are no more relationships between use cases than the above three relationships , and the use cases cannot communicate with each other.