What is SSG Platform? Is it a router, or NAT, or transparent proxi? Or is it transparent for network devices?

This platform is a DPI device similar to CISCO SCE. It operates as a bridge with no IP addresses assignment. It means it is invisible in a network. Its delay is no higher than 30 μs (16 μs according to tests). Therefore it is almost indistinguishable from the direct connection. Check SSG connection schemes for details