Curtain fullness describes density and gather of your curtain. More fullness gives better look.
Minimal fullness : At this level of fullness, curtain looks almost flat when they are closed. It has no waves and natural pleats.
Relaxed fullness : At this level of fullness, curtain has some waves and natural pleats when they are closed. It has medium curtain gather and density. This level fullness is most commonly used.
Luxurious fullness: At this level of fullness, curtain has many waves and natural pleats. Curtains look much gathered giving that exquisite look.
Pleated and Hook Top Panels have the fullness sewn into them, so be sure to fit the panels to your rod for proper coverage. For example, if you order a 59" hook top or pull-pleat, after processing final width will end up around 30".
Rod-pocket and Grommet Top do not have fullness sewn into them, For example, if you order a 59" grommet top or rod-pocket final width will end up around 59". But for grommet top curtains, after installing them on curtain rod entire 59" width won't be utilized to cover the 59" of window space. This will happen becaus eof the waves it will form after installing.
Click on the following link for information about different curtain top processing
Use following tool to calculate range of size of the fabric you need order to achieve your choice of fullness.