The variable transformer dials are marked as 0-100% of rated output voltage. It is recommended that the dial to be used for estimating the output voltage only. If the voltage needs to be set to a particular value then a separate voltmeter should be used. ISE offers optional digital or analog meters built into the case on many models.
The main reason that dials are not marked in volts is that the output voltage is a ratio to input voltage. Most of our models provide either 0-117% or 0-233% of input voltage (depending on the model). If your input voltage is not 120V (or 240V on 240V models) significant errors could result if just the dial is used.