The default values for things like freq and sustain are defined in the default Event. There’s a longer description of where this default event comes from here: How do Events get their defaults?
The short-short answer - freq, for example, is defined as:
freq: #{
(~midinote.value + ~ctranspose).midicps * ~harmonic;
}
It’s defined this way because (by default) it gets calculated based on other keys in the Event. Usually, you would specify \degree or \midinote since they are more musically meaningful - but you can also override this and specify a \freq explicitly as well. In your use-case, it should always be safe to call .value() on the arguments coming from the outer synth, so it’s probably better to get in the habit of doing this rather than trying to learn which are functions and which are not.