The fundamental expression for Rony's extent of separation in a 2-component system has been reexamined for any separation stage with respect to its relation with the traditional stage separation factor for a wide variety of separation processes. It has been shown that for a single entry or double entry rate governed barrier separation stage, the extent of separation is a composite index consisting of a stage capacity factor proportional to the barrier area and a stage enrichment factor (α − 1) or (β11). The latter-need not be close to zero. For a reverse osmosis and a gaseous diffusion stage, the extent of separation has been expressed in terms commonly used for these processes. The composite nature of the extent of separation has been also determined for batch dialysis, batch thermal diffusion, as well as for a single equilibrium stage, and related to known features of these separation processes in terms of stage capacity and stage enrichment.