Observations

Как — понятно: отдельный синхронный механизм (опционально). У оракла, по кр. мере.

Specify ORDER to guarantee that sequence numbers are generated in order of request. This clause is useful if you are using the sequence numbers as timestamps. Guaranteeing order is usually not important for sequences used to generate primary keys.

ORDER is necessary only to guarantee ordered generation if you are using Oracle Database with Real Application Clusters. If you are using exclusive mode, sequence numbers are always generated in order.

В другом месте они приямо говорят: "Selecting the ORDER option ensures that each NEXTVAL request across the cluster will be synchronized so that usage is chronologically ordered. This is achieved by having all sessions share a single cache for this sequence. Each reference to NEXTVAL will involve a lock request to claim a new value".

Понятно, что для линеаризации в рамках одной сессии достаточно NOORDER и просто неповторяющихся чисел.