What is the difference of circle sign of sukun and semi circle sign of sukun?

Question
Selam.
What is the difference of circle sign of sukun and semi circle sign of sukun? Why do they have the same name? Thanks

Answer
Wa alaikum assalaam wa rahmatullah,
As far as the difference in meaning, there is none if you mean different copies of the Qur'an using different ways of representing the sukoon.  The addition of markings and vowels was done later by scholars, and not part of the original copy of the Qur'an at the time of the Prophet, peace and blessings of Allah upon him.  Some scholars prefer using the head of the without the dot to represent the sukooon, the standing for khafeef, or light.  Other scholars use the circle.   They have the same name because they are both sukoons. 

There are zeros used in the writing of the copies of the Qu'ran which represent letters not read or letters read only when stopping and we shouldn't get them mixed up with the sukoon.