There are some references to a new thing called ‘Nexus Protect’ on the Google Store. It doesn’t shows up easily, you have to dig into the page source find a message like this (and has you can see in the image above):
You can’t add this phone unless it has the same Nexus Protect choice as other phones in your cart. You can update your choice or purchase this phone in your next order.
It seems that ‘Nexus Protect’ should be like an extend warranty protection plan, something like to Apple Care the third-party electronics warranties sold by retailers. With this, maybe Google it’s defining a limit purchases based on Nexus Protect choices. However, don’t take this for guarantee, since there is only a few information about this subject this is only a speculation. Some Google retail programs like this pop up in official documentation and never appear.
This pop-up seems to appear on Chromebooks, Android TV, and Android Wear devices that aren’t made by Google.
If this turns out to be actually a protection plan, probably we will hear about that when the new Nexus phones from LG and Huawei are announced.