Reason for choosing 90 rays in ground reflection model?

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Are there any particular reason for choosing a distribution of 90 rays from receiving points and ground hit points in the calculation of solar diffuse ground reflection? And, looking into the fortran-code "SolarReflectionManager", it doesn't show the exact number of rays, but it doesn't seem like 90 rays is distributed from neither receiving points nor hit points. Does anyone know the exact number of rays that are used?