Think of it in terms of variability the slider only dictates how much full scale resistance your trainer utilizes.
Let me use the kickr as a the example. (For a 75kg rider the sim mode provides at full scale 15% grade resistance)
At 100% if you are a 75kg rider you will feel a full 15% grade at or above 15% beyond that it can not do more
On the other end if you are going down hill at a -15% grade there is like no resistance at 100% (this being said the tacx neo has a motor inside to propel you down a simulated 5%)
At 50% the max grade felt is 7.5% at 15 and above
At 50% the max downward resistance is not full release