Hi Keith,
It might be that you found the information in a quite old whitepaper from release 2.0B called "Extracting Sales and Distribution Transaction from BW 2.0B". I have it stored as a local file in my archive and I found following information in it:
The extract structure MC12VC0SCL refers fundamentally to the “dynamic” LIS communication structure MCVBEL for delivery, in which the delivered quantities are dynamically assigned (that is, via program logic at the time of extraction) to the schedule lines that are based on the delivery item. In OLTP itself, there is no delivery item to schedule line assignment stored in the transparent tables.
The delivered quantities can only be transferred per schedule line as key figures in this extractor.
From PI 2000.2/PI-A 2000.2, the fields WMENG, BMENG, CMENG and LMENG of the order schedule line, will no longer be allowed (this change should have been effective with PI 2000.1/PI-A 2000.1, but was not included due to an error). Do not choose these fields under any circumstances. If you do choose these fields, their values will be transferred to BW several times for partial delivery and subsequent delivery of an order item.
Unfortunately I am not able to find it anymore on SCN and I cannot attach it as a Word document.
Furthermore please review SAP Note 574861 - BW Extraction (shipping): Restrictions to 2LIS_12_VCSCL DS.
Last but not least please refer to SAP Help.
Best regards,
Sander