The unit run is used to define the base value for all calculation of roof slope (the unit length). In the system here in the U.S. we define this value as 12 or better stated 12" for all common rafters. This value changes with regard to hip and valley rafters, in their case 17" (more accurately 16.97"). These two values work hand-in-hand for all 90° corners or structures that are square or rectangular.

In the case of other structures such as an octagonal form the unit run would still be 12" for all common rafters and 13" for all hips or valleys.

That this in effect does is give you a way to calculate the length of a hip of valley rafter by multiplying it's length by a know ratio. Knowing the length of a common rafter in a "square" structure you can just multiply it by (16.97/12) or 1.4142 (which is also the square-root of 2) to find the length of the corresponding hip or valley.

You can do the same with a octagon by multiplying the common rafter length by (13/12) or 1.0833 to find the length of the corresponding hip.