"Mapping" is a term popularized by by Mig Jimenez, since he defines it on page 101 of his book; F.A.Q. Frequently Asked Questions of the AFV painting techniques.
"Mapping represents all those irregular patches that can be detected on a tank but by nature are not really chips, but rather irregular, randomly created stains." The term is borrowed from CAD program terminology, where textures and colors are "mapped" onto a 3-D object to provide color and textural layers to the finished design.
Mig's technique involves roughly blocking out areas for mapping with thinned acrylic paint, applied in layers until the desired effect is achieved and allowed to dry, then blending the effects with oil paint washes and powered pigments.