Chlorine stabiliser runs your pool. It is the single most important chemical level to be able test for and dip strips can’t accurately test it.
The vanishing dot type test is in expensive and good enough for most people’s needs.
Packaged pool chemicals Multi action galetts, lent and choc all contain chlorine stabiliser (cyanuric acid CYA)
Cheaper galetts are usually dichlor, the more expensive are trichlor, that means adding 2 part chlorine (Di) or 3 parts (tri) this means, For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, (HTH chlorine sticks usually) it also increases Calcium Hardness (CH) by at least 7 ppm
You need some CYA to protect the chlorine from being burned up by the sun, the usual level required is 30-50ppm.