What are the definitions of upper arc and lower arc in mathematics?
Optimal arc: An arc with a central angle greater than 180 is called an optimal arc.
Bad arc: An arc whose central angle is less than 180 degrees is called a bad arc.
Simply put, an arc larger than a semicircle is an optimal arc, and an arc smaller than a semicircle is a suboptimal arc. There is no good or bad semicircle.