The Structure function combines the texture with a multifractal pattern. This pattern is synthesized from a Variator map.

Select the pattern source at the Structure panel. You can enable the Random Offset checkbox to have all layers of the pattern synthesis displaced randomly. Disabling the checkbox results in more regular patterns.

The number of fractal layers is controlled with the Octaves parameter. Increase the Persistence value to strengthen the details of the resulting pattern. Use the Resolution control to define the size of the pattern.
Change the contrast of the pattern with the Amplify value.

If you set the Bump value to zero, the pattern will directly variate the brightness of the target texture. Values other than zero emboss the texture, with the pattern used as height profile.

When using the Structure function in embossing mode, control the direction of the light source at the Direction panel.