For the most part, it's just specific support for specific workstation applications that AMD can't release publicly due to some contractual reasons or that those specific changes would be a detriment to a general selection applications. There are a few OpenGL features in there not in the open driver. There's also the DirectGMA thing for having hardware DMA directly to and from the GPU without CPU involvement.

The latter I wish was just a general purpose feature now that things like resizable bar are seeing general support in the consumer space.