People often equate sharing APK files with piracy, but there are many situations where it's perfectly legal and acceptable. In those cases, you still want the APK to retain its authenticity so the Play Store recognizes it as a legit application. Google is introducing a new system for marking "apps obtained through Play-approved distribution channels" as "distributed by Google Play."
As it turns out, Google's been thinking about what they can do with this problem, and now they've finally found a solution. Last year, Google announced a slew of updates to app security. Today, they have expanded on that by adding small security metadata on top of developer's uploaded APKs in the Play Store. This will make sure that an APK distributed from Play Store will always be displayed in the Play Store, even if it was shared while offline.
As Google explains, they took the measure to make sure that developers reach a wider audience. They said that peer-to-peer application sharing is popular in countries where mobile data plans are expensive and not available for all. The following change will give people more confidence that an application is definitely from Play Store. Apps downloaded from peer-to-peer sharing will be added to your Play Store library (assuming they originated from the Play Store).
Another noteworthy change is that app authentication can now happen in offline mode. This means that you won't need any kind of internet connection for making sure of applications authenticity. Developers don't really need to do anything other than uploading an APK file, and that's nothing new for them. New metadata is now built into APP Signing Block. Google Play's maximum APK size will also be increased to take the added metadata into account.