Thea allows the creation of an Infinite Plane that can be used as Ground Plane in the scenes. It doesn't have geometric properties and it is employed only as shadow catcher. The correct way to add a Ground plane in Thea Render is by creating a Floor Cinema 4D object and apply a Shadow Catcher material to it. This particular material has been designed to cover most of ground plane cases and of course it can be modified as any other material. It contains a Glossy layer, which is used for reflectance purposes and it has the "Shadow Catcher" parameter enabled in its General tab. Thea Render can have only one Ground Plane, so if there are multiple floor objects in Cinema 4D's hierarchy, only the first one is transferred into the engine.