Not sure if Variable control is the right terminology but here goes:
When you look at the preset for the TAL J-8 that’s in the preset list, there are a lot of controls doubled. This is because with one button press in the plugin you can go to to a the upper and lower panel. It would seem more efficient to be able to assign virtual controls. If I’d add a pad control and give it two states (upper & lower) and then add another control, say the fader control for cutoff, changes value (and even type, etc) dependent on the value of the pad control.
I plan to eventually make an instrument file for my Korg Prophecy, it would be quite handy there as well. It has hundreds of parameters, so there are probably not enough pages to give each control a place in the preset. For instance, the oscillator parameters depend on the oscillator type (or actually a combination of 2) you choose. In the synth the parameters are immediately changed so you don’t have for instance a “pluck type” parameter for a regular VCO.
I hope that’s a bit clear, otherwise I’ll go more in depth with visuals, graphics and whatnot…
Maybe it could piggyback on this idea:
https://forum.electra.one/t/make-parameter-number-independent-from-midi-cc/474/7
If value of controlid_1 (panel) = 1 then controlid_2 (cutoff) = 21 (lower) else 22 (upper)