You will find 2 different implementations for this, the first (very wrong) is a unique sequence for every table and it serves the purpose of a HIST_SEQ column.

The second (correct) is a global sequence which will be the same for all records in all tables which are updated by a single transaction. The purpose is to make it trivial to find all records (inserted, updated [ and deleted if using _A tables]) in a single transaction. [You’ll want to add an AUDIT_UNIQUE_TRANS_ID column to your _A tables for that linkage]

In simple environments this can be just a simple sequence and in more advanced environments this can be a UUID. The key is it must be unique on every transaction but its value should not be used to provide any information about the order of events in a table (that is the job of a HISTORY_SEQ column).