What's the difference between the "Base SDK for all configurations" and the "Active SDK" in Xcode?
The "Active SDK" value is effectively a build-time override of your "Base SDK" value, and determines which version of the SDK is used to build your application. In general, you should build your application against the latest stable SDK (at the moment, that's the non-beta 3.0 SDK), and set your "iPhone OS Deployment Target" to the version of the oldest device you intend to support (e.g. 2.2.1).
来源：https://stackoverflow.com/questions/1109609/xcode-iphone-base-sdk-active-sdk-difference