SAP manages BOM component substitutes by assigning the component and all of its substitutes to an alternate item group. Each BOM can have many alternate item groups. The alternate item groups have a two-character alphanumeric identifier, which must be unique on the BOM.

When maintaining Windchill substitutes on a SAP system, the correct alternate item group must be provided. It would be possible to relate alternate item group identifiers to BOM line numbers. However, in the case where BOM line numbers are not used, the alternate item group numbers in a BOM must be saved when the BOM is created or updated so that future maintenance activities have access to the correct alternate item group identifier.

Suppose that an assembly, 100-A, has two components: 200 and 300. Assembly component 200 has a substitute 400. Component 300 has no substitutes (see the figure below).

Any one assembly can contain multiple components, as shown in the figure below. The same assembly can also contain multiple instances of the same component. The components are attached to the assembly by WTPartUsageLink objects. The WTPartUsageLink objects may have line numbers, as shown below. If line numbers are used, there is one alternate item group for every line number that has a substitute.

The WTPartUsageLink objects may not have line numbers, as shown below. If line numbers are not used, there is one alternate item group if one of the links between the assembly and the component has a substitute.