Assuming you currently have less than 16 destinations assigned ...

When you are editing any parameter - changing the setting by hand - you will have "selected" this parameter as the one you are currently editing. The selected parameter will change value typically if you turn the data dial or press the [INC] [DEC] buttons. Note that when you have selected any parameter - you can look at the [CONTROL ASSIGN] button to see if MODX allows assigning this parameter as a destination for modulation control (controlled by the source as an expression pedal, for example). The button will illuminate for parameters that can be assigned and will not be illuminated if the parameter is not a valid destination.

You can also look it up in documentation - but this would take longer than having the keyboard tell you.

I say "assuming you currently have less than 16 destinations assigned" because the maximum number of destinations you can assign per PART is 16. When the destinations are all assigned you are at maximum capacity. Under this condition, the [CONTROL ASSIGN] will not illuminate - even if the parameter is a valid destination.

Thank you for such a helpful response. So. the first time I find a parameter I really want tied to the super knob it turns out I can't. Yamaha, REALLY?

There's a truck load of parameters you can program values for that you cannot assign as modulation destinations. I think Montage/MODX had a 5 pound bag with 10 pounds of stuff to fill inside - so they had to make some choices of what to include and what not to.

Now keep in mind that "release" in the EQ/FX area is not really a parameter. These are offsets to the programmed values and do not change the programmed values.

That said - the MODX data list ( https://usa.yamaha.com/files/download/other_assets/2/1192592/modx_en_dl_b0.pdf ) page 175 shows all of the destinations in a table titled "Controller Box Destination". "Element AEG Release Time" is a parameter you can change. I'm not sure where, in the interface, you're editing the parameter. You need to be sure you're editing a parameter like the Element AEG Release Time and not an offset like the value of one of the EQ/FX knobs.

That's not to say this does what you want either. Level (attack, decay1, decay2, release) are not able to be assigned as destinations. Only the time value. So if level is what you need (individual level of one stage of the ADSR) - then this is not an available destination. The EQ/FX "release" parameter is indeed TIME and not level. Therefore - you should be able to get this programmed to superknob.

The complexity is more about the difference in a single knob (EQ/FX section) which can offset multiple elements at one time and how you would need to make your own modulation assignments - one for each element you want to affect. Which could be "a bunch" potentially. There's not a way to modulate the EQ/FX section knobs - so you have to drill down to the level of the parameters this knob controls (each element - within the context/mode set by the keyboard) and make all of those assignments. If you have a simple 1-PART Performance with only one element - then there should be no more complexity. If you have 8 PARTs with 8 elements each PART - and you want to change the sustain time on all - it make take a while to do all of these assignments.

Note that scenes can change a more common release offset. So leveraging scenes may be something you can explore if you do not need a dynamic/fluid change in the release time. If you need a few different discrete values that have some breathing room between switching back/forth - scenes may work for you. Given your brief description - I do not think this will do for what you are doing - so element release time assignments is the way to go.