What is the definition of "Unknown source" for APK?

When you try to install an APK, not from the Play Store, you would need to allow your phone to install from unknown source. I'm looking for a technical definition of "unknown source". - Does it refer to an APK that is not signed by the Play Store's private key? - Does it refer to an APK that is not signed at all? (Is that even allowed?) - Does it refer to an APK that is signed with a default Android test key? It would be especially great if the answer goes into the actual Android code that does this "unknown source" judgement.