This attribute is used to annotate when a type or member of a type was introduced, deprecated, obsolete or is unavailable.    Thisis done on a per-platform basis (currently Mac OS X and iOS).

The information is only accurate for active versions of the operating systems, the information is removed as soon as operating systems are deprecated or no longer supported by Apple.

It is the managed equivalent of Clang's availability __attribute__, which is the underlying mechanism that Apple uses to perform these annotations.