The behaviour of the Reduction Period (CUSTOM_T437S-HORIZ) is not clear.
The described behavior is defined in the below source code:
HZENDE = BUDAT + T437S-REDHZ.
MOVE HZENDE TO PLAF_SEL_DAT.
SELECT * FROM PLAF INTO TABLE PLAFZ
WHERE MATNR = MATNR
PWWRK = WERKS
AND PLWRK = PLWERK
AND PEDTR <=
AND SOBKZ EQ SPACE
AND KBNKZ = SPACE
PLSCN = SPACE
AND SOBES NE ‘V’
AND VRPLA EQ space.
If you would like to reduce a particular planned order by backflushing in the transaction MFBF:
Enter the planned order number in the Section:
Without entering any planned order number, the system would select all open planned orders valid until the backflush date (normally the current date) plus
the reduction period maintained in the REM profile. This means that all the planned orders for the assembly will be taken into account when reducing the
production quantities if the order finish date is equal or smaller than current date + Reduction horizon days. This includes planned orders in the past, even
when you have planned orders dated on the same day than confirmation date.
See more details about this topic in the note 1932420 .
Sorry the link was wrong , now it should work.