Steam quality and steam purity are topics which often get confused with each other. Distinguishing the difference between the two will help you understand the unique impacts they have upon a steam system.
Understanding steam quality requires understanding vapor-liquid equilibrium in water. As heat is added to subcooled liquid water, it proceeds to the saturated liquid curve and becomes a saturated liquid (moving from incoming city water to point A). At the saturation point, any heat added to the water will form a vapor and the temperature will remain the same because all additional enthalpy (heat energy) being added to the system is being used to change the phase of the water from liquid to vapor.