Depending on the speed class of the device (which is the same as the Vivado part) you use to generate the project, the ISP core clock has a specific threshold (600 MHz) above which it operates in overdrive mode (0.88V).
When the ISP0/1 core clock for any of the ISP tiles crosses 600 MHz, the wizard displays
the warning Voltage overdrive necessary, ensure VCC_MMD voltage is 0.88V by
selecting Enable Overdrive in "Initial tab" of Configuration Options
advising you to enable the overdrive option in the wizard, which consequently sets the
operating voltage in the background.