Dome/Cap nuts serve two main purposes: (1) as decorative pieces, and (2) as covers for projecting threads
A castellated nut, also called a castle nut or slotted nut, is a nut with slots (notches) cut into one end. The name comes from the nut’s resemblance to the crenelated parapet of a medieval castle.
Slotted nut, is a nut with slots (notches) cut into one end.