FDA PQ/CMC Project needs to track the various impurities for an API.  These impurities are substances themselves and when they exist, we need to know things like name, graphic, data file, format, analysis file, etc.  Same kinds of things like a Substance.  There are some additional elements we need when a substance is an impurity

Impurity Classification: A categorization of impurities based on its origin.

Coded element - Values: Process- related, Product-related, Contaminants

Process Related Impurity Category: A classification of impurities derived from the manufacturing process. [Source: ICH Q6B]
Example: host cell proteins, antibiotics, leachables, etc.

Coded element:  Values: Organic, Inorganic, Residual Solvent

Product Related Impurity Category - A classification of impurities that are molecular variants of the desired product which do not have properties comparable to those of the desired product with respect to activity, efficacy, and safety.  [Source: ICH Q6B]
Examples:   precursors, certain degradation products arising during manufacture and/or storage.

Coded element: Values:  Product-related Impurity, Degradants

Proposed update to the SubstanceDefinition Resource:

1. Consider "Impurity" as a SubstanceDefinition.Relationship.type
2. Add the above 3 elements in SubstanceDefinition as Optional

Let us know if this would work.

Thanks!