Ordered delivery
Data is delivered in the same order it is sent and a message m is delivered only after all messages sent
before m have been delivered. (The corollary to this is that if message m is lost all messages after m will
be lost.)

"Sent" is just that something has been despatched. 例如发送邮件用的就是sent，东西没有直接给对方，而是给了第三方，再由第三方给对方
"Delivered" means that the goods have arrived at their desitnation and have been received.