PDME — Define the Substitute Usage model

Source: JPDM team (jpdm.team@mscsoftware.com)

Summary:
In 8.3.19, the intent of the Substitute relationship is clearly defined, but the rules for its usage are not. In particular,
section 8.3.19 should specify that a Usage that plays the "base" role in a Substitute relationship cannot play the "substitute"
role in a different Substitute relationship, or provide some other mechanism for disambiguating "base" Usages. In creating a
unique Bill of Materials, it is necessary to have a conceptual "primary" or "base" Usage that is distinct from all "substitute"
Usages. It is possible that there are multiple "base" Usages for different mutually exclusive Effectivity contexts; and it is
possible that the base Usage for one Effectivity context is an admissible substitute Usage in another Effectivity context. In
order to guarantee consistent interpretation of assembly models, the specification must address these aspects of the object
model.