Hi Kenji;
Thanks for the replies and the information shared.
The first solution that we wanted to implement was to operate the automatic creation of the outbound IDoc reservation when we transmit an Inbound Idoc for to create reservation.. The goal was to retrieve the reservation number in order to use it in another flow. After several searches, we have chosen to use the BAPI instead of the IDoc. So can transmit our reservation number using a external range number. This works well.
I should mention that flow that we put in place is between an external system and SAP using an EAI.
If need more information, do not hesitate.
Best regard