Uses a combination of a mask texture and tiling color, and normal maps to apply micro-detail to an existing material.
This technique, while greatly beneficial to detail density, is incredibly picky about having great input data. Meaning, when tiling detail textures 10x10 times across a surface, and local maxima/minima in luminance or grain size will be perceptually magnified, betraying the repeat. For this reason, creating detail textures with as homogenous of content size/clarity/luminance/distribution is paramount.
If you are finding your material darkens overmuch when using Mul mode, try using MulX2, or brightening and desaturating your detail color map.
Lastly, remember that with detail texturing, subtlety is key. Always start with authoring incredibly light/weak color/normal maps for your detail set, and increase their intensity only once you've tested them. Otherwise you'll end up with materials that look absurdly bumpy, or artifact from an overabundance of surface noise.