>>>but i not fully sure about this because i heard sometimes that the system has an "internal logic" that all message of a particular scenario are enqueued in the same queue, no matter if the QoS is EO or EOIO.
I don't think this is true. In case EOIO no other messages will get processed if the first message is not successful as all the messages reside in same queue.
>>>let assume i could define a queue for one interface in PI EO_OUTBOUND_PARALLEL/A = 50 but the backend only support 10, the messages that were sorted in PI will be enqueued de-sorted in backend because the difference of number of processing queues.
In a way Yes. If the receiving system doesn't have as many work processes(inbound queues) as the sender then they'll just be waiting in PI. Call adapter step may consume more time though mapping and outbound binding steps would be executed quickly.