In This scenario the sender system used in TRFC to transfer information to the receives server
this meaning that all the information that transfer to the too the recives server will executed one time only
you cannot define the sequence in which the function modules are executed,
nor the time of execution.
If an error occurs during the transfer, a batch job is scheduled, which sends the function module again after 15 minutes
qRFC with outbound queue
In This scenario the sender system used in outbound queue to serialize the data that is being sent his means that function modules
which depend on each other (such as update and then change)
are put into the outbound queue of the sender system,
and are guaranteed to be sent to the target system one after each other and one time only.
The received server has no knowledge of the outbound queue in the sender system meaning that in this scenario,
every SAP system can also communicate with a non-SAP system.
qRFC with inbound queue (and outbound queue)
In This scenario there is also outbound queue in the sender system and inbound queue in the recivied system
If a qRFC with inbound queue exists,
this always means that an outbound queue exists in the sender system,
This guarantees the sequence and efficiently controls the resources in the client system and received system
the inbound queue can process many data is the system resources
in the recived system at that time allow.
This prevents a server being blocked by a client A scenario with inbound queue in the server system is not possible,
since the outbound queue is needed in the sender system,
in order to guarantee the sequence and to prevent individual applications
from blocking all work processes in the sender system.
This blog is my summary of the SAP article