What do the answers of the API VK method mean?

Question

Python

Hello!
I use the VK API method auth.checkPhone and set the auth_by_phone field to 1 to check if the number is registered in VK.
In response, I receive either 1 or 2 or 3 (no explanation, just a number), the answer is 2, as I understand it, the number is registered, I can assume that 1 is not registered, but what is 3? The official documentation does not say, support refuses to speak. Tell me, who knows?