What is #define UIKIT_STRING_ENUMS ？

the code:

<UIKit/UIApplication.h> line 293-297
 #if UIKIT_STRING_ENUMS
 typedef NSString * UIApplicationLaunchOptionsKey  NS_EXTENSIBLE_STRING_ENUM;
 #else
 typedef NSString * UIApplicationLaunchOptionsKey;
 #endif


<UIKit/UIKitDefines.h>
 #define UIKIT_STRING_ENUMS ((defined(SWIFT_SDK_OVERLAY_UIKIT_EPOCH) && SWIFT_SDK_OVERLAY_UIKIT_EPOCH >= 2))

What is #define UIKIT_STRING_ENUMS ？