The Application Program Interface (API) is installed with the application by default.
API versions are named by the minor version number, and indicated stable API signatures for the duration of that version. For example, the 9.x API is the programmatic interface starting with version 9.x. The signatures present at that release remain unchanged throughout the life of the 9.x API. Additional signatures, if added during the 9.x.y release, remain stable during the lifetime of the 9.x API.
Backward compatibility with Version 4.x is not supported. Also, two different versions of an API in a release is not supported.