Parts per million (ppm)
A ratio rather than a quantity. For CO2 it is by volume: 1,000 ppm means 1,000 CO2 molecules for every million molecules of the surrounding gas.
The unit says nothing about how much gas is in a room, only how concentrated it is. A 1,200 ppm setpoint in a small tent and in a 40 by 20 ft flower room represent very different quantities of CO2, which is why sizing a supply starts from room volume rather than from the setpoint.
Converting a concentration into grams needs the volume of the space and the density of CO2 at room conditions. That calculation is what sits behind any estimate of what a room will consume.
The same abbreviation is also used for dissolved solids in nutrient solution, where it means milligrams per liter and has nothing to do with the gas. Two unrelated measurements sharing one abbreviation causes regular confusion.
The reference points worth holding: outside is roughly 420 ppm, a commercial flower setpoint sits at 1,000 to 1,200 ppm, and OSHA's permissible exposure limit is 5,000 ppm as an eight hour time weighted average.