The flat head is mainly used to draw some modern works. Fan-shaped pens are mainly used in classical painting and super-realism.
The round head is also easier to draw. Used between modern techniques and classical. Fan-shaped pens are usually medium size. You can buy two, with a smaller capillary.
The flathead can be 3 pieces each. There are also several round heads of different sizes. Portray the details to the minimum. A large pen is required for the paving background.