An Ultralight arm generally consists of two arms segments and three clamps giving you the greatest flexibility. Videographers sometimes use just one arm segment and two clamps. They do not need the same amount of flexibility as a still photographer does.
The o-ring in the balls of the arms, base adapters, and strobe adapters eliminates the metal to metal contact. This allows a much more linear clamping force, allowing you to control the arm movement when used with small or large strobes even in varying current conditions.
The ball size of Ultralight arms and adapters is 1" or 25mm.