Color Space ROP
...Specify that this ROP should be used to define the color spaces for TX conversion.
LDR Color Space
If it exists in the OCIO config, this should be set to the name of the sRGB color space. This is used internally for input and output color spaces in auto mode for low dynamic range images. If set by the user, this color space is also used as a reference to detect the rendering color space gamut and white point.
Rendering Color Space
This is the default linear color space that Arnold will use as its rendering color space. Arnold's default color space is sRGB Linear but this can correspond to any linear color space if needed. If chromaticities for this linear color space can be guessed or are user-specified certain spectral effects will take them into account, but not other adaptations for albedos, transparencies, etc are applied.
More information about color management can be found here.