Merging with Global Material BOMs 
Consequence of Fixing for Configurable Assemblies
If you fix an order BOM that is processed with variant configuration (CU51), the order BOM can still be re-read and re-displayed. When you load the structure with this function, the changed order BOM is merged with the structure that already exists for the order. Changes affect all configurable assemblies in the structure, whether or not they have been opened in this step. When you save, this merged structure is written to the database.
Note
If you have fixed non-configurable assemblies, they are not merged with global material BOMs (which may have been changed in the meantime).
Order BOMs and manual changes are merged according to the following rules:
Any item that was changed in the order BOM (change to any data field) is not changed by the merge. Manual processing overrides the merge.
Any item that was deleted in the order BOM remains deleted in the merge.
Any item that was added to the order BOM remains in the order BOM after the merge.
Any item that was added to the material BOM is in the order BOM after the merge.
Any item that was deleted from the material BOM is also deleted from the order BOM by the merge, unless it was changed manually beforehand.
Any item that was changed in the material BOM is copied to the order BOM after the merge, unless it was changed manually in the order BOM beforehand.
How can I prevent a merge?
If you want to prevent merging of order BOMs for configurable assemblies when using Order BOM functions, you must write a user exit that defines the circumstances under which the order BOM is not updated to match the model. (This requires a field entry in the header of the BOM for which you want to prevent an update to match the model.)
Processing order BOMs with Variant Configuration (CU51)
You can use variant configuration functions to process order BOMs for non-configurable materials in the same way and with the same result as if you were using the function without variant configuration (CS62).
You only need to process configurable assemblies in the transaction for order BOMs with variant configuration if you have suppressed merges with global BOMs, or if you want to use merges with the effects of changed characteristic values.
If you want to be able to use the options for reassigning characteristic values (with or without merges), do all of your order BOM processing in this transaction.
If you assign new values to characteristics in an order BOM that has been changed manually, the new configuration result is merged with the manual changes according to the rules described above, then with the global order BOM, unless you have used a user exit to suppress this.
If you only use result-oriented order BOMs, and only use configuration to generate an initial result that is then subject to extensive manual changes, it is best to use CSAB with order BOMs without configuration (CS1/62).
See also:
Merging order BOMs without variant configuration: