Bolting a UVB tube in for the last weeks of flower is supposed to push a CBD plant into making more resin as a sunscreen, and the theory is sound though home results are mixed. Too much UV burns the leaves and cooks the terpenes, and the cheap reptile bulbs people use vary wildly in output. A summary of the few controlled tests on CBD strains, most of which found a small effect at best, is on https://calmcbdseeds.com (https://calmcbdseeds.com). I would call it optional and risky rather than a secret weapon.