You may be confused by the term Quality Region to describe the region between the saturated liquid line and the saturated vapor line. This region is usually referred to as the Saturated Liquid-Vapor Mixture Region however we prefer the shorter form Quality Region, which was introduced by Potter and Somerton in their Schaum's Outline of Thermodynamics for Engineers.
The term Quality refers to the dryness fraction of the steam as shown in the T-v diagram. below. This usage came about because of the effect of the steam on the turbine blades of steam turbines. A low quality steam will result in the condensed water droplets causing deterioration due to erosion of the turbine blades, and one should always try to maintain a quality of above 90%. One example of the effects of this erosion can be seen on the blade tips of the final stage of the Gavin LP turbine. During 2000, all four LP turbines needed to be replaced because of the reduced performance resulting from this erosion. (Refer: Tour of the Gavin Power Plant - Feb. 2000)
Engineering Thermodynamics by Israel Urieli is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License