ATG 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 all spikes on the CPU — caused by new grass patches coming into sight when your camera is moving.
Next to this initial purpose ATG offers significant improvements regarding lighting and wind animation.
Since version 1.2 ATG supports the Universal Rendering Pipeline which also supports touch bending or grass displacement.
Since version 1.07 ATG supports third party solutions such as Vegetation Studio and lets you use ATG's advanced lighting and bending features within these.
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.
Documentation / Official Thread