This is nozzle efficiency. It is used in the calculation of zone run time on flex schedules. The higher efficiency the nozzle, the lower the actual run time is since less water is being lost. Drip systems obviously have a much higher efficiency than other nozzle types. Our defaults are generally close to industry standard per nozzle type. I'd generally leave this value unless you know exact specifics for your nozzle type.
Here is some more information on the topic.