4.2.7 Occlusion Culling
Occlusion Culling is a process that disables the rendering of objects when they are obscured from the view of the camera. This saves GPU processing time by rendering fewer objects.
Unity automatically performs frustum culling when objects exit the camera
frustum completely however, depending on your style of application, there might still be
other objects that cannot be seen and do not have to be rendered.
Unity includes an occlusion culling system called Umbra. For more
information on Umbra see occlusion culling in the Unity documentation.
The settings you use for occlusion culling depends on the style of your
game. You must be careful picking settings because using occlusion culling in a scene
with the incorrect settings can degrade performance.