One is for MirrorView sync and one for MirrorView async – which work differently.  See the MirrorView knowledgebook for more details
Hello all  I'm not sure of the difference between the following 2 statements. It seems to me that the result is the same, can anyone clarify please  With MirrorView/S, writes continue to the primary image but are not replicated to the secondary during a fracture. Replication can resume when the user issues a synchronize command.  With MirrorView/A, the current updates stop during a fracture, and no further updates start until a synchronize request is issued. The last consistent copy remains in place on the secondary image if the mirror is to be updated.
MirrorView Fracture 