The system landscape contains all the SAP Systems that you have installed. It can consist of several system groups, whose SAP Systems are linked by transport routes.
SAP recommend a three-system landscape in which each of the central clients has its own SAP System.
This consists of a development system DEV, a quality assurance system QAS and a production system PRD.
Make all changes to DEV client. When you release the corresponding change requests, they are transported into the quality assurance client. In the quality assurance client you can test whether the transports are complete,
If the test is successful, the change requests are transported into the production client. The production client is completely separate from the other clients as regards cross-client data.
The two-system landscape does not include a separate quality assurance system QAS. The quality assurance client is also in the development system DEV.
As in the three-system landscape, the production client is completely separate from the other clients. The disadvantage of a two-system landscape is that cross-client data is used in both the Customizing and quality assurance clients. This means that any changes that are made to cross-client data in the Customizing client can affect the tests in the quality assurance client. You can also not guarantee that transports from the Customizing client will be complete. Although all tests in the quality assurance client were successful, errors could still occur after the transport into the production client. This problem is caused by changes being made to cross-client data and then not being transported.
SAP do not recommend a one-system landscape containing all central clients in a single SAP System. Joint usage of hardware resources and cross-client data places serious restrictions on how a single system operates. In particular, once the system is used productively, you can no longer develop in it, unless you stop productive operation for the development and test phases.