The term "mapping" refers to making a map of correspondences between a source domain and some other "target" range. (Think of the game where you are given words in one category and are challenged to try to find an appropriate correspondence in another category, as in "Kitten is to cat as puppy is to ...".) The simplest kind of numerical mapping is called "linear mapping". That's when a one-to-one correspondence is drawn from every value in a source range X to a value that holds an exactly comparable position in a target range Y.
This example shows the patch from “A subpatch suitable for use in poly~” being used as a subpatch inside the poly~ object. You will need to download the patch from that example and save it with the name "FMsynth~" in order for this example to work.
A linear fade-in or fade-out of audio is okay for quick changes that take place in a fraction of a second, but for slower fades one should generally take into account that our subjective sense of loudness more closely conforms to an exponential change in amplitude. This patch allows you to use the slider (or the mod wheel from a MIDI controller) to control the amplitude of white noise in one of three different ways: with straight linear mapping, exponentially (with an exponent of 4), or using a range of 60 decibels.