fsno is the snow cover calculated based on the amount of snow. fsno_eff is the value used in calculations such as the soil temperature routine. When subgrid fluxes is turned on, they are equal. When subgrid fluxes are turned off, the snowpack is assumed to be uniform over the gridcell, and fsno_eff = 1. In that case, fsno is used to calculate the albedo, but for other calculations, fsno_eff will be used.
