Yes, that's right. To be fussy it is an "upper bound commit SCN" with respect to the cleanout of a particular transaction, but the "control SCN" with respect to its rollback segment. However, they are the same thing really. Yes, an ORA-1555 would be returned immediately if a query needs a consistent block image prior to the upper bound commit SCN of a previously cleaned out transaction.

Just to clarify my understanding, is the upper bound scn the earliest possible scn that can be rolled back to in a particular rollback segment. Referring to your example, if another query visited the block and this query had a snapshot scn prior to the upper bound scn would Oracle immediately produce the ORA-01555 error?