No new subtype children may be added in an extension.
Please refer to the following pages for guidance on the addition of subtype descendants.
| All SNOMED CT metadata concepts are subtypes of [900000000000441003 | SNOMED CT Model Component (metadata)|](http://snomed.info/id/900000000000441003) and include concepts, descriptions and relationships that are used to describe or provide additional information about SNOMED CT content and reference sets. The table below lists the subtypes of the [900000000000441003 | SNOMED CT Model Component (metadata)|](http://snomed.info/id/900000000000441003) hierarchy, and describes the content within these subhierarchies. **Table: Subhierarchies of |SNOMED CT Model Component| and their intended purpose** | Subhierarchy | Purpose | Link to More Information | | ------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **Core metadata concept (core metadata concept)** | Provides structural information required to support International Edition data. This supporting information includes sets of enumerated values that apply to attributes of concepts, descriptions and relationships. | [Core Metadata Hierarchy](https://docs.snomed.org/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) | | **Foundation metadata concept (foundation metadata concept)** | Provides supporting metadata and structural information for *Reference* *Sets*. | [Foundation Metadata Concept Hierarchy](https://docs.snomed.org/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) | | **Linkage concept (linkage concept)** | Links two or more concepts together to express compositional meanings. All *concept* codes that can be used as a *Relationship Type* are included under \[ | [Linkage Concept Hierarchy](https://docs.snomed.org/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) | | **Namespace concept (namespace concept)** | Contains *concepts* that represent an assigned *Extension* *namespace identifier*. | [Namespaces](https://docs.snomed.org/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) | | **OWL metadata concept (OWL metadata concept)** |Contains concepts that are used in
Content for the OWL Ontology Refset
Content for the OWL Axiom Refset
| The following pages provide information about the subtypes of [900000000000441003 | SNOMED CT Model Component (metadata)|](http://snomed.info/id/900000000000441003) . Each page includes tables which describe the purpose of the content within one of the sub-hierarchies, with examples from the International Edition. The tables also state rules about the additions that are permitted in an extension to the respective subhierarchy. Where additions are permitted to the subhierarchy, notes are added on practical uses cases for addition, potential problems that may arise from these additions and any other relevant considerations. * [Core Metadata Hierarchy](https://docs.snomed.org/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) * [Foundation Metadata Concept Hierarchy](https://docs.snomed.org/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) * [Linkage Concept Hierarchy](https://docs.snomed.org/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) * [Namespaces](https://docs.snomed.org/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) Provide Feedback --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.snomed.org/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.md?ask=