There is a following question on the web with different answers:
You must synchronize the starting point for Replicat with the restore point of the target database. How should you accomplish this? A. Use the command option AFTERCSN. B. Use the command option ATCSN. C. Use the command option UNTIL CANCEL. D. Use the command option RESUME.
There are more than one correct answer, i think.
A is correct, if datapump has been used to instantiate the target database. expdp performed consistency as of FLASHBACK_SCN included.
B is correct, if RMAN has been used to instantiate the target database. Oracle RMAN restore or recovery using UNTIL SCN which specifies an SCN as an upper limit but not including the specified SCN.
For Oracle GoldenGate 12.2 and above, neither AFTERCSN nor ATCSN is required anymore, if “DBOPTIONS ENABLE_INSTANTIATION_FILTERING” is enabled in replicat parameter file.
Reference : Oracle GoldenGate Best Practices: Instantiation from an Oracle Source Database
Document ID 1276058.1