I am fairly new to Script Runner Behavior and trying to write scripts with the help on examples available online – I must be missing something very obvious. 
Based on the value Feature Request Category(FRC), mandatory/ optional settings of Model field must vary.
I have used script runner to dynamically mandate Model when FRC value is “Missing Special Test” and optional when FRC value is “Missing Vehicle Model”.
Model field is getting displayed as optional when Missing Vehicle Model is selected for the first time, but when the value of FRC is switched to Missing Special Test (where Model field is mandatory) and back to Missing Vehicle Model – Model field is not automatically switching to optional.
How to make it optional every time Missing Vehicle Model is selected?
Here are some screenshots -
