After the release of Motion Studio, it became clear that the already powerful retargeting system needed more attention. With the old retargeter you would export your retargeted rig to a game engine for example. You would need to import a T-Pose from Motion Studio as well as another file with the animation data.
Now with the new system, you can just have your original rigs inside the engine and just import the exported motions from Motion Studio. Motion Studio automatically calculates your co-ordinate space of the rig and ensures it will match correctly on re-import into your favourite engine. No more hassles of retargeting in another program. Motion Studio does it all for you.