'NATS' is 'Network Address Translation with Sub-Address' in short. It is enhanced NAT which is widely used in the Internet. On residential network, especially, NAT is used for sharing many network terminals such as PCs using only one IP address.
Generally, there are two types of NAT. One is NAT, other is NAPT. When people use for sharing many terminals using one IP address, it is said 'NAPT'. This document never mentions NAT and NAPT.
As this document mentioned above, NAT/NAPT is used for sharing many network terminals using only one IP address. However, when people uses NAT/NAPT, they use private IP addresses which are never used in the Internet in residential network, generally. And, those IP addresses are translated to global IP addresses, when a terminal which is connected the residential network accesses to the Internet. This translation technology makes some problems.
For example, when web server is placed in residential network using NAT/NAPT, it can not be accessed from the Internet without special configuration.
For improve those problems, NATS protocol defines 16 bit sub-address space for each IP address. If a terminal uses sub-address, when a terminal can directly connect to other terminal which is placed at residential network through NATS router.