# 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**       | <p>Concepts added in these hierarchies should conform with:</p><ul><li>Concept model rules applicable to the relevant concept domain</li><li>Quality standards applicable to all SNOMED CT clinical content.<br></li></ul><p>These concepts should be classified to generate a set of inferred relationships in necessary normal form, for inclusion in the extension distribution files.</p> |

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