Well,
They underline the existence of a 4th type of shaders that seem supported only on these cards and not exposed in any API. So we have GS/PS/VS…
But does anyone knows what ES stands for?
And where it would take place in the pipeline?
And why we couldnt have an ATI proprietary extention to expose it with GLSL? 
Source: http://www.x.org/docs/AMD/R6xx_3D_Registers.pdf
Cheers,