Group GameObjects together in a lightmap with Baked Tags

The two images below 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
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In this example, all the GameObjects have the same Baked tag, and Unity stores all lighting data in one atlas. In this example, one GameObject is assigned a different Baked tag, and Unity creates a separate lightmap for it.

Select the CPU or GPU for baking

Troubleshooting baked lightmaps

Unity Manual

Group GameObjects together in a lightmap with Baked Tags