> For the complete documentation index, see [llms.txt](https://docs.atfinity.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.atfinity.io/rule-language/operators/special-operators/is.md).

# is

### Description

Only used in [declarations](/rule-language/declarations.md), `is` assigns a name or identifier to the specified [ontology](/guides/glossary/ontology.md). This name or identifier can then be used in the same rule to refer to that ontology, when you want to access [information](/guides/glossary/information.md) on it to use in the condition, or to add a [field](broken://pages/-MLC7Nuj2H4vw8p5A0EZ) on a [document](/guides/glossary/document.md).

### Example

```
p is Person
```

A declaration with `is` is needed before you can have any expressions using `p`, like: `p.domicile != ch`.