Variant BOM
A variant BOM groups together several BOMs that describe different objects (for example, products) with a high proportion of identical parts.
A variant BOM describes the specific product variant for each product, with all its components and assemblies.
If you are producing several similar products that have a lot of common parts, you can describe these products using a variant BOM. This is the case, for example, if you replace one material component with another to make a different product. Variants can also differ by containing different quantities of a component. You create the new BOM as a variant of an existing BOM.
You can only create a variant BOM from a simple material BOM. No multiple BOM can exist for the material.
A multiple BOM cannot be converted to a variant BOM.
Multiple BOM
A multiple BOM groups together several BOMs that describe one object (for example, a product) with different combinations of materials for different processing methods.
One product can be manufactured from alternative combinations of materials depending on the quantity to be produced (lot size). The product is represented by a number of alternative BOMs (alternatives). The differences between the alternative BOMs are only small. Usually the only difference is in the quantity of individual components.
Multiple BOMs are only supported for material BOMs.
Hope it is clear
see the link also
http://help.sap.com/saphelp_47x200/helpdata/en/a0/1016bd4c8311d189520000e829fbbd/frameset.htm
