Wondering if there is a safer way to identify which parm is being affected by a 'control' parm besides the obvious string pattern of '<parm_name>_control'? Is there something in the API for a control parm that indicates which parm it's paired with?

The relationship actually goes the other way... Each value parm can have a "usdcontrolparm" spare parm tag to point to the parm that should be used as the control parm. Other than that, it isn't actually the name of the parms that matters, but the relative positions of the parms... Control parms affect all value parms that follow it until the next parm that ends with "control".

Got it! Thanks Mark!