The deployment target specifies the lowest operating system version that your app can run on.
There are several strategies for choosing the deployment target when developing your app. Each version of an operating system includes features and capabilities not present in earlier versions. As new versions are published, some users may upgrade immediately, while other users may wait before moving to the latest version. You can target the latest version, taking full advantage of all the new features but limiting the app to only users running the latest version. Or you can target an earlier version, making your app available to more users but limiting the features you can use in the app. Another approach is to target an earlier version but use weak linking to determine at runtime whether later version features are available before using them.
The SDK version, not the deployment target, determines which features you can use in an app. If the SDK you’re using to build the app is more recent than the app’s deployment target, Xcode displays build warnings when it detects that your app is using a feature that’s unavailable in the deployment target. Ensure that the symbols you use are available in the app’s runtime environment.
For details on weak linking and to check the available symbols, read SDK Compatibility Guide.