PPF is photosynthetic photon flux and measures the total amount of PAR that is produced by a lighting system each second. This measurement is taken using a specialized instrument called an integrating sphere that captures and measures essentially all photons emitted by a lighting system. The unit used to express PPF is micromoles per second (μmol/s).
So this is probably the second most important way of measuring a horticulture lighting system but for whatever reason.
Most lighting companies don’t list this metric and its important to note that PPF does not tell you how much of the measured light actually lands on the plants, but is an important metric if you want to calculate how efficient a lighting system is at creating PAR.