NAT (network address translation) - (see Wikipedia) is the process of re-writing the source and/or destination addresses of IP packets. Most systems using NAT do so in order to enable multiple hosts on a private network to access the Internet using a single public IP address.

What is a diffrence between NAT implementation in TrafMeter and third-party NAT implementations?

The integration of NAT and TrafMeter solves very important task - an accounting "clear" traffic of NAT router (without traffic of users on a private network). So, if you have a job place on a computer acting as the Internet gateway with running TrafMeter, you can count the traffic of the Internet gateway without the traffic of other uses from the private network.

TrafMeter NAT engine doesn't require specific IP addresses for the private network (in difference with "Internet Connection Sharing" service of Microsoft Windows 2000 Professional, Windows XP and Windows Vista which assign the IP address to the private network interface).