Order Preservation

• If the backend guarantees order, the messages are received in the precise order in which they were sent. For example, if message M1 was sent before M2, you will not receive M2 first and then M1.
• If the backend does not guarantee order, you very well may receive M2 before M1.

If backend is a multi-tenant distributed system, order preservation may require extra effort and cause performance penalty. That is why, say, SQS provides this property as an optional feature for extra pay.