润色：The observed velocity shift of BALs are explained by several possible causes 1The actual acceleration or deceleration of the outflowing material in the direction of the observers line of sight 2 th

However, the limited number of observation cases does not necessarily mean that the physical mechanism causing the velocity shift of BALs is rare or uncommon. It is possible that many BALs with velocity shifts have not been detected yet due to the limitations of current observational technology and methods. Therefore, more systematic and comprehensive studies are needed to explore the physical mechanism of BAL velocity shifts and to understand the properties and evolution of outflows from quasars and active galactic nuclei