I want to add Today extension to my app with base SDK iOS 7.0. Extension should work on iOS 8 devices and just shouldn't appear on iOS 7 devices. I know that app extension can work only on iOS 8. But it exists as a different target in my project. So can I release the app if the containing app will have base SDK 7 and app extension will have base SDK 8. I have tested it and everything works just fine. Today extension appears in Notification Center only on iOS 8 devices and it doesn't appear on iOS 7 devices, no errors. But sometimes everything works fine in sandbox, but can cause the issues submitting to the App Store. So will this work?
The answer is NO. Your base SDK should be 8.0 or 8.1 in order to deploy a containing app with an extension. But you can set the deployment target to 7.0 so your app runs on iOS 7 too. Base SDK is the SDK you use to build the app. Deployment target is the minimum supported iOS version you want.
Your containing app should have entitlements too (app groups) which are not supported in SDK 7.0.
Apple has posted a guide on how to support older iOS versions with a containing app that deploys extensions.