I was confused as well and asked him. This is his response.
Mapping -- this term is actually more literal than a lot of people suspect. Think of a geographical map where the contour lines (of maybe a hilltop) are related and receding from the previous line, but they don't overlap.
With mapping, you are working with the same base color range on top of the previous layer, and not fully covering it with each successive layer. You can also switch between types of paints too, oil on top of acrylic, or enamel of acrylic too. Or thinned acrylic on top of acrylic base coats works great too.
Whitewash is a classic example perfect for this technique; burnt out rusted vehicles too. Layer 1 has the most coverage, layer two is applied on top (usually with a brush, and any paint type can be used) and covers maybe 75% of the area, adding more opaque layers on top without fully going back over all of layer 1. Layer 3 is more of the same, but only covers maybe 50% of layer 2... and so on.
It's a great way to build up opacities, and depth on certain types of finishes. See this T-34 whitewash below and notice how the lower whitewash layers cover the most area, the brighter more opaque layers on top, less so.
