Installations of the Web Application Server ("landscapes") typically consist of three systems: one for development; one for testing and quality assurance; and one for production. The landscape may contain more systems (e.g., separate systems for unit testing and pre-production testing) or it may contain fewer (e.g., only development and production, without separate QA); nevertheless three is the most common configuration. ABAP programs are created and undergo first testing in the development system. Afterwards they are distributed to the other systems in the landscape. These actions take place under control of the Change and Transport System (CTS), which is responsible for concurrency control (e.g., preventing two developers from changing the same code at the same time), version management, and deployment of programs on the QA and production systems.