An unloading curve applies to pumps, fans, cooling and heating equipment. It is used to determine the power consumed when the device is operating at part load (i.e. a 1000 ton chiller operating at 400 tons requires an unloading curve to determine it's efficiency at 400 tons)
Example:
A fan with a VFD, that has a design supply of 1000 cfm and 300 watts, consumes only 90 watts when operating at 300 cfm. In energy modeling, these numbers are determined from an unloading curve.