Depending on the choices made as part of a SNOMED CT implementation project, the addition of expressions to an expression repository may be done at various stages of implementation and by different users. For example: 

• Some implementations may restrict the creation of expressions to only during the implementation stage, where
  • Expressions are created by people involved with the implementation
  • Expressions are added to the expression repository to be made available for end users in the same way as precoordinated SNOMED CT content. By this approach, the end users can not  themselves directly create new postcoordinated expressions
• Some implementations may enable runtime creation of postcoordinated expressions by end users, where
  • Expressions are created by end-users as part of the data entry process
  • Expressions are added to the expression repository as part of saving patient data in the EHR record store.