When you add terrain details using only a 2D grass texture then Nature Renderer does not have any specific material to use for that texture. By default, Nature Renderer will create a new material (internally) and assigns the 'Open World Nature/Grass' shader to that material.
If you want to use a different material then you can assign a custom material to the Billboard Grass Material property in the Nature Renderer component. This material is then assigned to all the terrain details that use 2D grass textures. The main texture of the material is replaced with the grass texture, and all other properties of the material are kept the same.
2D grass textures are a convenience feature for quickly and easily adding terrain details. It is recommended to use a 3D model for more control over the detail's settings or materials.