What does the filter cutoff value mean?

In Syntorial, the filter cut-off knob has values from 0 to 1. On some synths the value goes from something like -4 to +4. I've seen knobs labeled 20hz to 20kz.

In DSPs, a filter has a "significant" frequency where the action of the transfer function happens. In the formulas, this is expressed as a frequency (e.g. 440hz = A4) but if you look at the details, you divide by the Nyquist frequency (half the sampling rate) and translate it into (what I think of as) a percentage of the expressible frequency spectrum. So, if you're sampling at 44.1kHz and your cut-off is at an A6 or 1760, then the value is 1760 / 44100 = 0.039.

Now, we arrive at my question... what does Syntorial's [0, 1] mean? It definitely doesn't mean the % of the spectrum. Does it mean the % of the MIDI keys frequency? So, an A5 is MIDI note 81, so that's 0.64, so if the knob is set to 0.64, then the filter cut-off is at A5 (or 880hz)? For filter knobs that are labeled in hz, it is obvious what it is. And maybe when the knob is labeled [-4, +4] it is saying that it will do key-tracking and the cut-off frequency will be the note plus/minus the octave?

And relatedly, the filter envelope value knob, is it the target or is it the increment? So, if the cutoff is 0.3 and the env is 0.5, does the filter go from 0.3 to 0.8 or does it go from 0.3 to 0.5? I think it is an increment, because otherwise the env knob would never have any meaning when its value was lower than the knob (i.e. 0.8 and 0.3 wouldn't make sense). [I get that the env knob can "top out" because the cut-off value saturates at 1.0. So (0.8, 0.3) sounds the same at the end as (0.8, 0.2), but they sound different in-between, because the slope is different.]