Each of PotterDraw's properties can be modulated by an oscillator. Each of these oscillators has its own properties, such as waveform, frequency, amplitude and so on, and these properties can also be modulated. In other words, you can modulate a modulation. This means there are two levels of modulation in PotterDraw: primary modulation, in which an oscillator directly modulates one of the pot's properties, and secondary modulation, in which an oscillator modulates a property of a primary modulation.
Secondary modulation is an advanced feature that lets you create more complex effects. Its most common applications are amplitude modulation (AM) and frequency modulation (FM). For example you might want to apply a waveform to a property, but only on part of the pot. This is amplitude modulation, and it can be visualized as applying an amplitude envelope to a waveform. For another example, you might want the waveform's frequency to vary over the range of the pot. This is frequency modulation.
Other more esoteric types of modulation are supported, such as pulse width modulation (PWM), phase modulation, etc. Mutiple-choice (drop list) properties such as waveform or motif can also be modulated, but note that because they're integers they will modulate in discrete steps rather than continuously. Multiple types of secondary modulation can be applied to the same primary modulation, for example you could modulate a primary modulation's amplitude AND its frequency.
Secondary modulations are accessed via the modulation type drop list. This setting affects what's shown in both the modulation and oscilloscope bars. When modulation type is set to "Property", you're accessing the target property's primary modulation, but if it's set to any other value, you're accessing a secondary modulation. Note that for a secondary modulation to have any effect, the primary modulation it applies to must be enabled, i.e. its waveform must be set to something other than "None".
Example
Suppose you're modulating the Scallop Depth property with a sine wave, so that the scallops undulate as they progress vertically up the pot. Now suppose you also want to vary the intensity of that undulation, so that it's strong at the bottom of the pot, and gradually weakens to nonexistent at the top of the pot. In other words, you want to apply an amplitude envelope to the undulation. More specifically, you want to modulate the amplitude of the Scallop Depth modulation with a descending ramp wave. You could do this as follows: