When importing a variable, it gives you the choice of the units. Internally, PVsyst will compute the right multiplier to obtain the unit needed for simulation.

If you are recording data with a data-logger, you might want to add a multiplier to your measurements to obtain the right unit (e.g. if you are measuring a reference cell that has a certain tension per irradiance - V / W/m2). This multiplying factor will be applied to your data, in order to obtain values in the units waited by the program.