Chunked LOD- it takes each subdivision and applies LOD to it, using a similar system to the current entity LOD..
As has been stated many times by JCL, with modern GPUs, the cost of dynamically tesselating terrains usually outweighs the benfits. Chunked LOD is a good compromise, because its rarely noticeable, will reduce polygons counts by a good amount, and doesnt take any processing power to calculate.
I dont doubt that in the future, everything will be dynamically tesselated from higher-order surfaces, including terrains and all other geometry.. but that's not here yet with A6.. maybe A7.