Traditionally cyanuric acid (CYA) has been used in pools to help protect the free chlorine from the effects of UV light. In the ’80s and ’90s, you would often see operators going up to 50ppm in the quest to save money on chlorine costs when the sun came out. Later it became more accepted that the 20-30ppm range allowed a more reasonable ORP level to be maintained while still providing some protection. As CYA bonds with the chlorine, it takes away some of its ability to be an effective oxidizer/disinfectant.
More recent studies out the American Chemical Society have again put conventional thought about CYA on its head. These have found that in the event there is a fecal incident at the pool CYA levels where some users now operate don’t allow the hyperchlorination to properly clean the water of Crytpo etc.
As an example at 100ppm CYA, 7.6PH @ 25C it would take 40ppm Cl2 nearly 72 hours to achieve a 3log reduction in Crypto which is the standard to look for.
Even with 15ppm CYA in the pool, it is advised in the Model Aquatic Health Code to raise the free cl2 to 40ppm for 8.5hours
Thus it seems to make more sense to run your CYA around 16ppm to achieve the maximum beneficial effects from the CYA. While you will burn more chlorine it will lower your downtimes after a fecal and/or avoid having to drain part of the pool to achieve a CYA ppm concentration low enough for hyperchlorination to be effective in a reasonable amount of time.