The unstated idea is that you chrono with .2s, and then game with whatever weight you want.
As Styrak said, certain places will not allow you to use heavier weights. The reason for this, is that even though you have a slower muzzle velocity, the heavier mass of the BB allows it to hold onto that energy for longer.
In a very exaggerated example with made up numbers, say you owned a CQB facility where lightbulbs are exposed, 20 feet off the ground. If someone accidentally shot a .2 straight up at 400 FPS (~1.5 joules), it might lose a lot of its energy on the way up, and only impact at 300 FPS (0.8 joules of energy). If you shot a .3 straight up at 325 FPS (~1.5 joules), the mass would cause the BB to lose less of its energy to wind resistance, and might impact the light bulb at 300 FPS, conveying 1.25 joules, which may shatter the light bulb.
So the reason for chronoing at .2 is because that's the arbitrary industry standard that is used when calculating joules (you could chrono at any weight, since it's just one variable in a formula). But the reason you may not be allowed to go higher, is because the mass of the BB would allow it to retain the energy further out from your muzzle.
In most cases, that's not an issue, especially outdoors, so generally you can assume that you can use any weight for actually playing, unless otherwise stated.
Edit: ninja'd by Drake
Last edited by FirestormX; September 12th, 2014 at 15:12..