> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](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/5.3.2.1-top-level-concepts-and-clinical-concepts.md).
# Top Level Concepts and Clinical Concepts
## Top Level Concepts
| **Hierarchy** | [138875005 \| SNOMED CT Concept (SNOMED RT+CTV3)\|](http://snomed.info/id/138875005) |
| -------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------ |
| **Purpose** | The immediate subtype children of the root concept represent the most general types of SNOMED CT concepts. |
| **Concept Addition Rules** | It is strongly recommended that new top level concepts (i.e. direct subtype children of the root concept) are *not added* in an extension. |
| **Additional Notes** | |
## Top Level Clinical Hierarchies
| **Hierarchies** | All direct subtype children of [138875005 \| SNOMED CT Concept (SNOMED RT+CTV3)\|](http://snomed.info/id/138875005) except [900000000000441003 \| SNOMED CT Model Component (metadata)\|](http://snomed.info/id/900000000000441003) . |
| -------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Purpose** | These concepts represent a distinct type of clinical concept |
| **Concept Addition Rules** | Subtype descendants of the top level clinical concepts may be added in an extension. |
| **Additional Notes** | Concepts added in these hierarchies should conform with:
- Concept model rules applicable to the relevant concept domain
- Quality standards applicable to all SNOMED CT clinical content.
These concepts should be classified to generate a set of inferred relationships in necessary normal form, for inclusion in the extension distribution files.
|
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