# Constraints on Concept Requests

The SNOMED CT International Edition contains some concept hierarchies that have fundamental roles in supporting the design of SNOMED CT. Additions of concepts to these hierarchies may impact the consistency and integrity of SNOMED CT. The following pages set out guidance on the addition of extension concepts within specific hierarchies. In some cases, this guidance may prohibit extensions from adding concepts in a specific hierarchy or at a particular level of a hierarchy. In other cases, the advice is provided on the use case for making particular additions and notes are included on the implications and potential risks arising from such additions.

These constraints are expressed in tables in the following pages, which are arranged to follow the overall hierarchical structure of SNOMED CT.

* [Top Level Concepts and Clinical Concepts](/snomed-ct-practical-guides/snomed-ct-extension-guide/5-key-steps/5.3-assessing-requests/5.3.2-constraints-on-concept-requests/5.3.2.1-top-level-concepts-and-clinical-concepts.md)
* [Model Metadata Concept Hierarchies](/snomed-ct-practical-guides/snomed-ct-extension-guide/5-key-steps/5.3-assessing-requests/5.3.2-constraints-on-concept-requests/5.3.2.2-model-metadata-concept-hierarchies/5.3.2.2.1-core-metadata-hierarchy.md)
  * [Core Metadata Hierarchy](/snomed-ct-practical-guides/snomed-ct-extension-guide/5-key-steps/5.3-assessing-requests/5.3.2-constraints-on-concept-requests/5.3.2.2-model-metadata-concept-hierarchies/5.3.2.2.1-core-metadata-hierarchy.md)
  * [Foundation Metadata Concept Hierarchy](/snomed-ct-practical-guides/snomed-ct-extension-guide/5-key-steps/5.3-assessing-requests/5.3.2-constraints-on-concept-requests/5.3.2.2-model-metadata-concept-hierarchies/5.3.2.2.2-foundation-metadata-concept-hierarchy.md)
  * [Linkage Concept Hierarchy](/snomed-ct-practical-guides/snomed-ct-extension-guide/5-key-steps/5.3-assessing-requests/5.3.2-constraints-on-concept-requests/5.3.2.2-model-metadata-concept-hierarchies/5.3.2.2.3-linkage-concept-hierarchy.md)
  * [Namespaces](/snomed-ct-practical-guides/snomed-ct-extension-guide/5-key-steps/5.3-assessing-requests/5.3.2-constraints-on-concept-requests/5.3.2.2-model-metadata-concept-hierarchies/5.3.2.2.4-namespaces.md)

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