A cam company can say they have an ungodly duration cam by just saying it, so engine builders and racers wanted to know why one cam didn't run or perform as good as another cam companies cam when they were rated at the same advertised duration. So, they came up with measuring the duration between the opening and closing of the valve at .050" lift. This is when the valve first opens .050" and when it then closes to .050". .050" is lot when you look at a lot of street/rv/truck cams that have a gross lobe lift of only like .280" or less. This takes out any ramping of the cam lobes for different style lifters (or almost takes it out) or any grossly stated "Advertised Duration".
Also watch out for durations advertised at .020" lift. A few cam companies used to use this lift, not sure if any do now days or not.