Its called DLI or daily light increment.
What it means is that the plant wants a given number of photons for its daily needs. The plant will goto sleep even with the lights on and thus "waste" the light its given and that's why some people run 18/20 or whatever number of hours to save power.
This also means that with weaker light the plant will want more than sometimes given if running lower hours. I like to run 24 so the plant always has as much as it will take. Some people say roots grow best in the sleep period or dark and they try to force it by running 18/6. I dont find that to be the case for myself.
Anyways you'll find what works best for you, just sharing what I've learned/found.
