Coexistence and fallback play an important part in planning for migration to the latest release. This section explains what coexistence and fallback are, describes the IBM® policy regarding the releases that are supported for coexistence and fallback (as well as migration), and states which specific releases are supported.

While coexistence and fallback might at first seem unrelated, they are very much related in that both deal with an earlier level of a system being able to tolerate changes made by a later level.