Hi Richard,
I didnt try this myself, but it looks normal as described in SAP Help in transaction OS43 (see below) , meaning if you would like only to add 2 components (E, F) ,then you have to list all 6 (A, B, C, D , E , F) in order not delete the (A, B, C, D ) existing components
"Note
The input data must contain all items that are to be in the target system on the key date.
The report compares the items in the input file with the items in the taget system BOM on the key date, which you include in the BOM header of the input file. All items that are not included in the input file are deleted."
Regards,
Ziv