Android is one of the major Operating System distributed to many smart phone manufacturers. As the most distributed OS, Google now checks out whether the Android device officially certified or not. If the Android device is certified by Google then you are allowed to acces the Google official apps like Playstore, Movies, Maps etc. If the device is not certified, then you are not allowed to access the official apps by Google. Beyond this there are aslo some manufacturers who ship the product without certificate and permission from Google it seems.
If a user tries to access the Google apps, Google will check the build date of the Android system.It is to check whether the system is certified or not. If your running your device with Android OS that was compiled after March 16th, 2018, Google apps won’t run on that device.
As for the custom ROM users points out the good news is that they can now register their devices here. The user who customized their devices can maintain it up by registering their devices at Google. The limitation for the custom ROM users is upto 100 devices, as it seems to be maximum for the users to go with. As the users who has custom ROM can now officially get certified for their device. Making a official certified makes the custom users to go with personalize the device as official.