The special feature of "mixed mode" is that qRFC calls with send queue and normal tRFC calls are processed together within one LUW. For more information characterizing "mixed mode", see
qRFC With Send Queue: Overview.Prerequisites
See
Programming Serialization.Procedure
When implementing "mixed mode" within an LUW, note the following:
If within an LUW in "mixed mode" the first call is a normal tRFC call, you must first of all call function module TRFC_QUEUE_INITIALIZE. This assures that the current LUW is processed via qRFC with send queue.
If within an LUW in "mixed mode" the first call is a qRFC call, this initialization is done by function module TRFC_SET_QUEUE_NAME.

This program sequence is an example of how to implement an LUW in "mixed mode":
REPORT ... .
...
DATA Q_NAME LIKE
TRFCQOUT-QNAME .
CALL FUNCTION '
TRFC_QUEUE_INITIALIZE '.CALL FUNCTION 'RFC_FUNCTION_1' " tRFC call -> NO QUEUE
IN BACKGROUND TASK
DESTINATION 'DEST'
EXPORTING ...
TABLES ... .
...
Q_NAME = 'BASIS_TEST_Q1'.
CALL FUNCTION '
TRFC_SET_QUEUE_NAME 'EXPORTING
QNAME = Q_NAME.
CALL FUNCTION 'RFC_FUNCTION_2' " qRFC call -> Q1
IN BACKGROUND TASK
DESTINATION 'DEST'
EXPORTING ...
TABLES ... .
...
CALL FUNCTION 'RFC_FUNCTION_N-1' "tRFC call -> NO QUEUE
IN BACKGROUND TASK
DESTINATION 'DEST'
EXPORTING ...
TABLES ... .
Q_NAME = 'BASIS_TEST_QM'.
CALL FUNCTION '
TRFC_SET_QUEUE_NAME 'EXPORTING
QNAME = Q_NAME.
CALL FUNCTION 'RFC_FUNCTION_N' " qRFC call -> QM
IN BACKGROUND TASK
DESTINATION 'DEST'
EXPORTING ...
TABLES ... .
...
COMMIT WORK.
Result
Because of the transaction characteristics, the normal tRFC calls and the qRFC calls with the same destination form a unit. If, however, within the current LUW there are no qRFC calls for a destination, the system bundles the normal tRFC calls into a sub LUW.
The normal tRFC calls with the addition AS SEPARATE UNIT map a separate LUW in the target system.
qRFC calls with the addition AS SEPARATE UNIT are processed depending on the queue, but independent of the actual LUW.

To start programming in "mixed mode", refer to the demo program RSTRFCT1.