As you know, SCN, is a number internal to Oracle that is incremented over time as change vectors are generated, applied, and written to the Redo log.

There is also 'System Commit Number', also abbriviated SCN, which is a number also internal to Oracle that is incremented with each database COMMIT.

System Commit Numbers and System Change Numbers share the same internal sequence generator.

I do not know if this generator is cyclic.