> For the complete documentation index, see [llms.txt](https://docs.b2ihealthcare.com/snow-owl-authoring-platform/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.b2ihealthcare.com/snow-owl-authoring-platform/editing-and-authoring/evaluating-changes/validating-concepts.md).
# Validation
Validation within Snow Owl is executed on many levels.
* During import, the syntactic correctness and the referential integrity of the RF2 files are checked. The concept hierarchy is also validated to avoid loops.
* During the concept authoring, the editor is validating Fully Specified Name uniqueness and attribute conformity to the Machine Readable Concept Model.
* During [classification](/snow-owl-authoring-platform/editing-and-authoring/evaluating-changes/classifying.md), the ontology consistency is checked and equivalent concepts are detected.
### Validation during the editing process
To prevent the creation of invalid concepts, Snow Owl automatically validates your concept during the editing process. Conflicts are indicated by error or warning decorators. Hovering over the validation message reveals further information about the validation.
**Error decorators** 
inform about a fundamental problem within SNOMED CT, e.g. if your Fully Specified Name is not unique. It is not possible to save your new concept until the error is fixed.
**Warning decorators** 
indicate a conflict with the rules specified in the concept model. It's still possible to save the new concept despite this conflict. To review or modify the rules pre-defined in the concept model editor, click on the MRCM Editor button 
in the main toolbar to open the editor.
### Validating existing concepts
Existing concepts can also be validated. This is useful when want to ensure that concepts that you imported comply with the SNOMED CT guidelines. Erroneous concepts that were imported are indicated by an error decorator in the SNOMED CT View.
For QA rules that are not covered in the above three validation methods and a manual validation system has been implemented. Validation rules are based on the Expression Constraint Language (ECL), and as such, they are executed on the inferred hierarchy. Click the  icon to run validation.
{% hint style="info" %}
Validation is always executed on the active branch.
{% endhint %}
Components that fail validation are listed in the Validation Issues View, grouped by severity and rule number. Double-clicking a component opens it in the Concept Editor.
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