Well if it's a max effective weight (that's mass * acceleration) then setting X amount of weight on the absorber would result in it moving at a speed less than the maximum shock speed. Or putting that another way the weight would be a function of the max absorbed Nm and max shock speed. With a fudge factor, apparently.
This looks like Calculus. I'm getting out of here.
Well, I'd like to see the context, but I can take a guess at what that spec might be trying to say.
If you are using this shock as a buffer, that is it sits at the end of the railway track and absorbs the impact of a carriage running into it, then you need to know the max weight and speed of the moving object which you wish to bring to a controlled stop. If your idea of "high speed" is 2 meters per second and your thingy is 200kg then pick the 2020 version. If you think it might go faster, then the 2525 will stop your crashing weight in 25mm of travel.
Are there any pictures or diagrams in the datasheet that might show this test method?