Argument for multichannel expansion in a SynthDef

Yes, but are control signals also Ugens?

UGens are presented in much of the literature as the things that generate signals. But then in the above example the control signals appear to be also UGens. This is where I am confused. In @jamshark70’s modular analogy an oscillator generates voltage but the voltage is the result of the oscillator and not a subset of the type oscillator.