If I create iOS application in XCode 7.2 with SDK 9.2 using swift and set application compatibility to SDK 6, will that application work in iPhone 4s with iOS 6? OR is there any compatibility cheat sheet?
Swift will only work on iOS 7 and higher, it will usually crash at launch on older OS versions.
Here is apples short blurb on compatibility https://developer.apple.com/swift/blog/?id=2
App Compatibility Simply put, if you write a Swift app today and submit it to the App Store this Fall when iOS 8 and OS X Yosemite are released, you can trust that your app will work well into the future. In fact, you can target back to OS X Mavericks or iOS 7 with that same app. This is possible because Xcode embeds a small Swift runtime library within your app’s bundle. Because the library is embedded, your app uses a consistent version of Swift that runs on past, present, and future OS releases.