If you have a personal or company website, (or plan to create one for the app) then use that domain but reversed.
If not, you can use whatever you want, just have in mind that it will be used as package name for Android and as bundle identifier for iOS, so use something that relates to you and/or the app
So, if I had a web site called nonprofit.org, I could use org.nonprofit, or would I also need to attach the name of the app to it, org.nonprofit.testapp?
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I'll go with org.nonprofit.testapp as you might create more apps in the future.
But it's just an identifier, you can use whatever you want.
The same "reverse domain" syntax works for both iOS and Android.
The most important caveat is that you can't change it once set in iTunes Connect, so choose wisely.
Another advice is to always use the same preamble, like com.yourcompany, so that you can set up a Wildcard App ID for use with multiple applications.
And use a distinct and complete app name to make it easy to understand what the app does.
andersborglund, you can use * as wildcard App ID so it can be used for any app you build (unless it requires push, iCloud or some other features binded to the App ID where you need an explicit provisioning profile)
Understood. I thought it required a "root".