1. Compile-time checking OS version
This will check the maximum allowed OS version, that's the
Base SDK version that the project is using. So we can include some code that feature is only available in newer SDK. But if we always assume coders are using newest version of SDK, this directive is rarely used.
#if __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_6_0 -(BOOL)s