Many weld transformers are manufactured with a tap switch. This tap switch gives you the ability to change the turns ratio that you are using in the transformer. The net result is the output voltage changes up or down as you change the tap.
Reference: RWMA - Resistance Welding Manual 4th Edition
As the output voltage changes your ability to push amps to the part also changes. Higher voltages allows for higher total amps to be delivered to the part. The general rule of thumb is: