It's the highest SCN to which data is synchronised between the primary and secondary in a way that the secondary guaranteed read-consistent. It's therefore not possible to query an active standby for data modified after that SCN. At any given time, there might be transactions committed on the primary but not yet shipped or applied to the standby. You're not allowed to see those on the secondary. Not sure how much more detailed you want to get, really.
Edited by: Catfive Lander on Jan 15, 2013 4:15 AM