CGNAT (Carrier-grade NAT)
Updated
CGNAT is address translation done by the mobile operator: many devices share a few public IPv4 addresses, and each device only has a private or shared address. RFC 6598 reserves the range 100.64.0.0/10 for this shared address space.
In practice
Behind CGNAT a device can open connections to the internet but cannot receive one, which is safe by default but blocks remote access unless you add a static public IP or private delivery.