Your answers to your customer's questions should be simple to avoid confusing your customer. A simple question can help you find out if you need to provide more information. It also gives your customer a chance to have a discussion with you rather than hear a sales pitch from you. Keep it simple, then everyone will understand. If the voltage goes too high or low, the relay will drop out.
Applications usually involve the monitoring of single phase voltages.
The customer wants to know if the voltage goes to high or too low in order to avoid damage to their equipment.