Allows you to specify whether Unity should try to use 32-bit shadow maps, where possible.
Most platforms have a fixed shadow map format that you cannot adjust. These vary in format, and can be 16-bit, 24-bit, or 32-bit, and can also be either float or integer based.
However, when you are targeting PS4 or platforms using DX11 or DX12, you can choose whether Unity should use a 16-bit or 32-bit float shadow map.
32-bit shadow maps give higher quality shadows than 16-bit, but they will use increased memory and bandwidth on the GPU.
In addition, 32-bit shadow maps can only be used if the depth buffer is also set to 32-bit.