Decay Layer Controls
First off: You will almost always use Maximum mode.
Imagine a typical Field effect: a Field (e.g., Spherical Field) moves over a clone/object point and causes an increase and decrease of its effect values. If Maximum is selected, the increase will take place unchanged and only the decrease will be delayed. If Minimum is selected, the effect will be inverted - the increase will be delayed and the decrease will take place unchanged.
Define a value of 100% if no decay effect should take place, i.e., the maximum value is maintained. You can, for example, use it to "paint" because the respective value is maintained (see image here with an Effect Strength of 100%)
The decay effect will lessen in strength with decreasing values. A value of 0 will disable the effect.