An includes relationship is useful when you have a common set of steps that must be performed by many different use cases. Instead of copying the same set of steps in each use case, you can put those steps into a use case of their own, and then create an includes relationship to that use case from the others. Include relationships are depicted with a dashed line in use case diagrams.
To create an includes relationship to an existing use case, create an ID link in the steps or extensions of the use case. Note that for extensions, the ID link must appear in the steps of the extension (not the extension condition) for it to be considered an included use case.
To create an includes relationship to a new use case from existing steps, you can move selected steps into a new use case. The steps and their extensions, if any, will be moved to the new use case. The steps in the original use case will be replaced with an ID link that references the new use case.