LightDense is a special material in OE-Cake that can be taken advantage of to make more advanced creations. As the name implies, it is a mix of Light + Dense, but it is not equal to another neutral liquid like Water. The default Light + Dense neutralize each other's effect, causing the liquid to be neutrally buoyant and mix with Water evenly.
However if you change the Parameters upCoefficient and downCoefficient you can cause something different to happen. LightDense is only neutral when up and down are equal to each other. If up and down are made slightly different, they no longer completely neutralize and the material LightDense will have a slight tendency to move in one direction.
This allows 4 levels of density when mixing: Light, Water <-> LD, Dense, depending on what you set the Parameters to, instead of the usual 3: Light, Water, Dense.