NEW IN R21
Each volume already contains information about its density. This produces the visible shape of a clould and how dense it is at different locations. With increasing density, the volume will become correspondingly more massive and opaque. In addition, light passing throug it will become more diffuse and can even be prevented from passing all the way through. The density and its distribution are therefore important properties for the volume, which can also be adjusted using the Density Strength setting. This value works as a multiplier for the density value that is saved in the volume. In addition, a spline curve can be used to re-interpret the density value in the volume.
This function graph runs from the bottom left to the top right by default. Areas without density are at the bottom left and maximum density is at the top right. Manipulating the spline curve will redistribute the density within the cloud. A curve that runs from the top left to the bottom right, for example, will make previously transparent areas look mssive and make previously massive areas disappear.
Each of these settinga and how the function graph works is explained here.