Require Void Reasons
Navigate to Configuration > Security. About half way down the POS functions section you will see the "Require Reason on Voids". Check the box for each workgroup you wish.
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Click the Reasons button to add specific reasons. If you don't add a list, employees will be required to type a reason.
Once you've created your list of reasons, this is what will happen when you try to void in the POS.