The length of the cable is calculated as the overall length of the main trunk cable plus the length of all of the spur cables. This document assumes short spur lengths of 10 meters and the calculation for the main trunk should be sufficient in many cases. A spur is a drop off of the main trunk (the cable between the two terminators). If you are using longer spur cables, include their length in the overall calculation. If you have a choice about spur length, shorter is better. The total spur length is limited according to the total number of devices on the segment.