Material Requirements Planning in BOM Transfer 
Use
When you have assigned material components to network activities using BOM transfer and
activated requirements, the system produces requirements when you save the network.Features
You can explode a BOM top-down (multi-level) or just single level for the assignment. If you explode it for all levels, the system generates requirements for all levels in the BOM.
You can generate preliminary requirements for lower-level components by selecting both the Multi-level indicator and the Preliminary reqmnts indicator. You use the Preliminary reqmnts indicator to create requirements in advance which can later be posted against real requirements. To do this you must have made the appropriate settings in the material master.

If you have not generated preliminary requirements, and explode a BOM, excess requirements are generated, since bills of material are always exploded multi-level during material requirements planning.

If the higher-level assembly is an
In advance procurement the system generates:
MRP interprets a planned independent requirement as a normal requirement. Afterwards, when you generate a real requirement and the system finds a component for which a sufficiently large planned independent requirement exists in the same type of stock, then these requirements are set off against each other.
For more information on advance procurement and its prerequisites, see
Advance Procurement.