Azure Default Routes Explanation or what is the purpose of 100.64.0.0/10 IP range in Azure

I have simple question to Azure team. Is there any document which describes purpuse of each Azure default routes?

How Azure uses  100.64.0.0/10 IP range? 

I was playing with File Service and SAS tokens. I have created file servce and generated new SAS token with IP restriction. I restricted access to the single IP, which is public IP of the VM I deployed into the same subscription as this Files service. When I tried to access a file in this file service with provided URL, I got message access denied from the IP address. To my surprize the IP address which was listed in access denied message was not my public IP but  100.79.88.46. I fond that this IP belongs to the range of one of the Azure Default routs. ({100.64.0.0/10} Null Active Default )

If I go to this URL form VM in different subscription or from workstation I can see that my IP address is properly identified.