The WS-ReliableMessaging protocol is a SOAP-based RPC protocol for guaranteed delivery of messages; possibly in specific order from one sender to one receiver. Such messages are usual SOAP messages - XML documents conforming to the SOAP specification. The Sender is an alias of the transmission initiator, i.e. the originator of the message transfer. The Receiver is a recipient, that which accepts the messages. How accepted messages should be processed is not covered in this document.; What to do with the data and whether to send replies is at the discretion of the application.

Further in this section for brevity WS-RM will be used in place of "Web Services Reliable Messaging Protocol".

Delivery Assurances Types:

AtMostOnce - Delivery at most once without duplication or an error will be raised on at least one endpoint. It is possible that some messages in a sequence may not be delivered.
AtLeastOnce - Every message sent will be delivered or an error will be raised on at least one endpoint. Some messages may be delivered more than once.
ExactlyOnce - Every message sent will be delivered without duplication or an error will be raised on at least one endpoint. This delivery assurance is the logical "and" of the two prior delivery assurances.
InOrder - Messages will be delivered in the order that they were sent. This delivery assurance may be combined with any of the above delivery assurances. It requires that the sequence observed by the ultimate receiver be non-decreasing. It says nothing about duplications or omissions.
[Tip] See Also:

WS-RM System Table Definitions in the Appendix section