It's a mounting system that Trijicon offers - basically they have the lower mounts for a variety of weapons. These mounts attache to the 'upper ring' that is on the scope. These are the 'generic' Compact ACOGs.
The ones most of us buy have the built in AR-15 carryhandle mount and we need to purchase a mount to adapt it to the flattop (which IMHO is not quite as secure as the 'special ring mount'; M16 mount has 1 big screw to secure it while special ring has 4 smaller screws). If you go to Trijicon's website and click products, then Compact ACOG, then select a unit then click in 'Info'. The lower of the 2 scopes shown is a 'special ring' model - note the 2 screw holes on the side for attaching to the mount (there should be 2 on each side).
The problem with the special ring model IMHO is it costs an extra $90 to mount the scope to a M16 (even if it's a carry handle M16) and the scope is the same cost as the M16 ready model. Also with the Carry Handle mount there is no LaRue mount so you're stuck with thumbscrews (if that is important to you).