System requirements and environment are modeled in the analysis phase.
The environment not only includes the operating environment-- the external servers, hardware, and users modeled in use case diagrams-- but also includes the concepts and processes of the business environment, which are modeled in class and activity diagrams, respectively. The concept and process models often reappear in the design as the domain layer of the Multi-Layered Architecture pattern. The use cases often reappear in the design as the controllers in the Model-View-Controller pattern.