EP is not considered a pin, but in order to have a corresponding pad in the pcb, it can be modeled into a pin. In fact this is the chip base, put chips, used for fixed chips, also help to heat dissipation. Have not learned integrated circuit encapsulation, there may be some words is not accurate, don't mind. As to why is knowing this is stem what of, because when I was a child feel mysterious, chip wanted to see the appearance of the bare chip, has studied how to put the chip black sealant with the chip separation, cut\/fire many chips...