Group GameObjects together in a lightmap with Baked Tags

The two images above shows two views of the same SceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary:

1. Top: Everything is in one atlas because all the GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
   See in Glossary have the same Baked Tag.

2. Bottom: One GameObject is assigned a different Baked Tag, and forced into a second lightmapA pre-rendered texture that contains the effects of light sources on static objects in the scene. Lightmaps are overlaid on top of scene geometry to create the effect of lighting. More info
   See in Glossary.