So if I understand it correctly, the concept of Active Chlorine refers to the portion of FC that is actively sanitizing. So when we take a measurement of FC, we get a number, part of which is tied up with CYA, and not actively killing algae and pathogens. The remaining portion is the Active Chlorine. The amount of Active Chlorine is dependent on the FC level and CYA.
I've been thinking about this concept, and I'm curious if there is a noticeable difference in waters with higher Active Chlorine levels and lower Active Chlorine levels. Would there be a stronger chlorine odor with higher Active Chlorine? Will this water create the characteristics typically associated with "High Chlorine"?