To support complex communication procedures as simple request/response mechanisms, the sender is provided with the optional ConversationID tag for each message. All messages with the same ConversationID belong to the same communication procedure or integration process.
RefToMessageID is the MessageID of a previous message that is referenced by the current message. In the case of a synchronous response, RefToMessageID must be set to the message ID of the referenced message.
These tags can be found in XI Message Header.
Parts of an XI Message - SAP NetWeaver Exchange Infrastructure - SAP Library
Regards, Evgeniy.