Typically, a "relative" measurement simply means a ration of a value to the whole. For uncertainty, this would be (uncertainty)/(measured value). For example, if the test result is 10 kg +/- 0.2 kg, the relative uncertainty would be (0.2 kg)/(10 kg) = 0.02 = 2%.

Note that the units cancel, so relative uncertainty has no units. This means the original units don't matter and it is easier to compare various measurements. As a simple example, density requires measurements of both mass and volume, which obviously have different units. If mass has a relative uncertainty of 0.1% but volume has a relative uncertainty of 2%, then improving the mass uncertainty won't really improve accuracy of the density calculation, but improving the volume uncertainty would have a bigger effect.

Tim F