The RAS rail system on the real firearms is fixed on by the gas tube.
AEGs don't have gas tubes to connect both RAS and upper receiver.
RAS FIX is a new product which mounts on both the upper receiver and RAS. It will make your RAS and upper receiver connected as one piece for greater rigidity.
It can also be used as a scope riser mount.
Disclaimer: All aftermarket upgrade parts require necessary skill or modification to install and assemble, please consult with your airsoft technician if needed.