Co2 is obtained through the stomata of leaves. Most plants are not co2 limited until they reach the point of saturation where light levels are beyond what is possible for leaves to process at the current ambient level (around 400ppm). This has been shown by chandra to be around the 1000umols, where plants start becoming co2 limited. But it is more beneficial to use co2 supplementation when average light levels reach between 1000-1500umols as this is the point where severe co2 deficiency occurs and photosynthetic efficiency declines. Many might assume that using co2 will allow one to bring their lights closer and as a result, increase light delivery to plants. But this is not the case as increased light proximity does not provide more light to plants, due to the reason that their is no significant loss of light when light passes through air as a medium.
Where co2 supplimentation is most beneficial, is when more powerful light sources are used that is typically too strong for a typical sized grow area. For example, in a 4x4 where a 600w HID light is used. This provides an average light level of around 740umols of photon density. Which is right on the point where photosynthetic efficiency really starts to decline. So 600w in a 4x4 is the max you would want for that size grow area. Where as a 1000W DE HID, will have a average 1140umols of photon density. Way too much for a 4x4. However, with co2 supplementation, you can use these higher discharge light sources as the plant no longer will be co2 limited at these higher light levels.