A ball top is a sphere, usually 25 to 50 mm across, fixed to the end of a spear shaft. The simple version sits straight on the tube; a finial adds a collar and a short spigot under the ball so it reads as a turned piece. Balls are bought-in castings or turned aluminium, welded or pinned to the shaft, so they appear on the cutting list as a separate item with a quantity.
The shaft is cut to its own length and the ball is added on top, so the overall height of a ball-topped spear is the shaft cut plus the ball (and collar). Round tops are less aggressive than a point, which suits residential frontages, but they give up the anti-climb read of a spear, so check the use case before choosing one on a security or pool boundary.