It has two modes, variable wattage (VW) and variable voltage (VV). Most people use it in VW mode, its the default mode out of the box, and people just seem to use watts these days.
You set the wattage, the istick reads the resistance of your atomizer, and calculates the appropriate voltage to apply when fired. VV mode you just set the desired voltage, and fire (it also displays the wattage but is just there for the end user).