This is a feature of the Objective-C compiler rather than of the runtime, anyway since it is documented nowhere and its existence was forgotten, we are documenting it here.
The keyword @compatibility_alias allows you to define a class name
as equivalent to another class name. For example:
@compatibility_alias WOApplication GSWApplication;
tells the compiler that each time it encounters WOApplication as
a class name, it should replace it with GSWApplication (that is,
WOApplication is just an alias for GSWApplication).
There are some constraints on how this can be used--
WOApplication (the alias) must not be an existing class;
GSWApplication (the real class) must be an existing class.