When an application issues a write request:
the data is stored in a memory structure called memtable, and
persisted to disk by appending the mutation to a commitlog (WAL).
Appending to the commitlog ...
Perhaps a start on your journey to discovering this:
From the Oracle FAQs:
Every redo log file has both a log sequence number and low and high SCN.
Having said that, why do you feel you need to know ...