C3 photosynthesis
The carbon fixation pathway used by cannabis and most crops, named for the three-carbon compound it makes first. C3 crops gain from added CO2 because ambient levels leave them carbon-limited.
In a C3 plant, carbon dioxide is captured directly by RuBisCO and the first stable product is a three-carbon molecule. There is no mechanism for concentrating CO2 around the enzyme beforehand, which is what separates C3 plants from C4 and CAM plants such as maize, sugarcane and agave.
That absence is the whole reason CO2 enrichment works commercially. At the roughly 420 ppm found in outside air, RuBisCO in a C3 plant is running well below its capacity, and oxygen competes with carbon dioxide for the same active site. Raising CO2 in a sealed room tilts that competition toward carbon and suppresses photorespiration.
C4 plants already concentrate CO2 internally, so they gain far less from enrichment. Cannabis, being C3, is among the crops with the most to gain.