Hi @shibasou 

Use insert business rule,  simple script for copying variable value into additional comment--->

var theVariableValue = current.your_variable_name.toString(); 
    
    // Check if the variable has value
    if (theVariableValue) {
        // Adding the variable’s value as an additional comment
        current.comments = "Additional Comment: " + theVariableValue;
    }

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Thanks & Regards 
Deepak Sharma