Advanced Terrain Grass is a grass rendering solution which was built to replace the detail rendering system of Unity’s terrain engine in order to get rid of almost all spikes on the CPU — which usually are caused by new grass patches coming into sight when your camera is moving. So ATG will help you to reduce work on the CPU – but at the same time it might add some more costs on the GPU side.
Next to this initial purpose ATG offers significant improvements regarding lighting and wind animation.
ATG is not an authoring tool: It simply takes the grass and details you have added to your terrains and takes over their rendering. That being said ATG is compatible with any 3rd party authoring solution that allows you to create grass density maps for built in terrains within the Unity editor like e.g. Terrain Composer, Map Magic, World Creator or Gaia. Runtime distributed grass currently is not supported.