Axle rating is the weight limit permitted on an axle, or on a set of axles grouped together. It exists to protect roads and bridges and to keep vehicles handling safely, so it’s enforced by law rather than left to the driver’s judgment.
Even when a truck’s total weight is legal, an uneven load can put too much on one axle group and fail inspection. That’s why drivers weigh their rigs and, if needed, shift the load or the tandems to bring every axle within its rating before hitting a weigh station.