Photoperiod
The hours of light a crop receives in a 24 hour cycle. Cannabis flowering responds to the length of the uninterrupted dark period rather than to the light hours themselves.
Indoor cannabis generally runs 18 hours of light in veg and 12 in flower, but the dark half is the one doing the work. Photoperiod-sensitive cultivars initiate flowering in response to a long enough continuous night, and a light leak that interrupts that night can stall or reverse the process.
Autoflowering cultivars carry a trait that starts flowering on age instead of on night length, so they are not steered this way.
For CO2 enrichment, photoperiod defines the injection window. Carbon is only fixed while the lights are on, so gas injected into a dark room is not taken up by anything. That is the reasoning behind cutting injection before lights out and holding at zero overnight.
Photoperiod also sets the daily light integral together with PPFD. Hours multiplied by intensity is the total light the crop receives, and that total determines whether extra carbon has anything to work with.