Typically, dU/dV maps are thought of as derivatives (in the mathematical sense) of normal maps. A normal represents the normal (perpendicular) direction of a given point on a surface. A dU/dV map typically represents how much the normal is changing with respect to the U and V directions at any given point. This type of map is commonly used for approximating refraction effects by representing the distance for your shader to offset the UV lookup of an un-refracted frame buffer. It’s not DX8-specific.
Nvidia’s normal map generator is very well documented here: www.modwiki.net Nvidia Normal Map Filter