润色：The research on the physical mechanism that causes the velocity shift of BAL is limited by the scarcity of observation cases About two decades before the first report of the BAL velocity shift was

However, despite the limited number of observations, the study of BAL velocity shifts is crucial for understanding the physical processes occurring in quasars. The velocity shift of BALs can provide important information about the outflows from the accretion disks and the black holes at the centers of quasars. Therefore, future observations of BALs with velocity shifts are necessary to further investigate this phenomenon and gain a deeper understanding of the physical mechanisms behind it.