"X_area_fraction" means the fraction of horizontal area occupied by X. "X_area" means the horizontal area occupied by X within the grid cell. Cloud area fraction is also called "cloud amount" and "cloud cover". The cloud area fraction is for the whole atmosphere column, as seen from the surface or the top of the atmosphere. The cloud area fraction in a layer of the atmosphere has the standard name cloud_area_fraction_in_atmosphere_layer. The ISCCP cloud area fraction is diagnosed from atmosphere model output by the ISCCP simulator software in such a way as to be comparable with the observational diagnostics of ISCCP (the International Satellite Cloud Climatology Project).