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The substance redesign project has made a number of decisions regarding the representation of a substance and its modifications.

Modification indicates that the concept is a structural modification of another concept, where the intrinsic physicochemical properties of the substance do not change.
The intrinsic physicochemical properties do not encompass non-intrinsic characteristics of a substance, such as toxicity, bioavailability, or extended release (in the case of medicinal products).
Modification concepts that exist to group modifications or derivatives of a specific substance are not to be created.
Metabolite groupers in the substances hierarchy are considered roles. No further concepts of this format will be created in the |Substance| hierarchy. Existing concepts will only be retained where there is a specific requirement and will be modeled as a child of |Metabolite (substance)|. See Metabolites.
Structural groupers that reference modifications as a chemical group will be retained. Where the structural group name also refers to a specific chemical the grouper will be re-termed “Substance with X structure”. See relative section Substance Groupers Based on Structure.
There is no requirement to introduce a new semantic tag in order to distinguish concepts representing a substance or its modifications from any other type of concept in the |Substance| hierarchy.

Modeling

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The following illustrates the stated view of 125691000 |Sotalol hydrochloride (substance)|:

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Between a glycan and its glycoconjugate.
- For example, 259289005 |Trimethylene glycol (substance)| is not a modification of 52086008 |Glycol (substance)|.
To genetic engineering process variations (e.g. Somatropin(epr), Somatropin(rbe), or Somatropin(rmc) as SNOMED CT does not generally differentiate substances based on production process.
Between a chemical element and its salt.
- For example, 387307005 |Calcium carbonate (substance)| is not a modification of |Calcium (substance)|.