iOS deployment target

IF I set the deployment target for my app to iOS 4.2 when submitting to a开发者_开发问答pp store, will users with iOS 3.2 (or anything below 4.2) be able to download and install the app and the app won't run or will they not be able to download and install the app itself?

Thanks.

They won't be able to install and run the app (until they upgrade the OS on their device to that of the Deployment Target or above). But they might be able to buy and download the app using iTunes on their Mac or PC.

No, that's exactly what deployment version is about.

Best practice is to build against latest SDK version, with deployment version as low as possible. But that requires the discipline to do runtime checks and fallback implementations whenever using a API that wasn't there yet in the deployment version.