In the Main part field, you can link a selection alternative to an incorporated M-part in the BOM list in the basic preparation. The selection alternative will then affect the incorporated M-part's BOM list. If this field is empty, the selection alternative will affect the BOM list at the top level in the basic preparation. This can also be entered for selection groups configured as Optional.
If you use the main part field for selection alternatives, the part with the basic preparation should be locked to the configuration group in the Part number (locked to) field. You can then only select an M-part that is included in the BOM list in the basic preparation in the Main part field.
Fictitious Part as Main Part
You can use a fictitious part as main part to be able to select material to an M-part in Selection group when the material in not incorporated in the main part's basic preparation. Then the M-part should only have the fictitious part in the BOM list on the basic preparation. The fictitious part will then be set as main part for the different Selection alternative for selection group that can be selected to the M-part.