When modeling an application, you may encounter subject matters that deal with the same information but addressing different requirements. When the requirements are varied enough, it may make sense to model parts of the application on separate class diagrams… each with its own view of the data.
For example, here is a model of the syntax of a language. This model focuses on the content, form and inter-relationships of the elements of the language.
This language can be marked with options that affect its translation. Marking deals with the same information but is addressing a separate and substantially different requirement. So, a new view of the data is modeled that focuses on the requirements of providing the needed translation options. In xtUML, this is done with an imported class (or class reference) that is depicted on more than one class diagram.
