what's the meaning of Set Depth Image Scalar Type To UChar,UShort(other than Float)should be used at what situation?

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what’s the meaning of SetDepthImageScalarTypeToUnsignedChar and SetDepthImageScalarTypeToUnsignedShort, other than SetDepthImageScalarTypeToFloat?
isn’t DepthBuffer normally use float? if use UChar/UShort, how to decide the depth value?
Thanks!