there is a paremeter in lcd controller datasheets,which is a fraction like 1/8 or 1/16 , they call it "lcd duty" waht does it mean?

for example for the famous Hitachi 44780 controller we can define this parameter using "function set" instruction;

Function Set 0 0 0 0 1 DL N F * *

when n is 0 => 1/8 or 1/11 Duty (1 line)
and when n=1 => 1/16 Duty (2 lines)

how does this setting affect lcd operation?
thanks,Koosha