This specifies which PotterDraw property the Modulation bar is currently affecting, also known as the target property. It contains a drop list of all the properties that can be modulated. You can also change the modulation target by left-clicking a property name in the Properties bar.
Most target properties are floating-point values, but enumerated values such as Scallop Motif can also be modulated. Note that the following properties can't be modulated: Rings, Sides, Texture Path, Colors, Color Sharpness, and Palette Type.