When you work with the API, you may notice two similar fields in API entities:
`id` and `iid`. The main difference between them is scope.
Some resources have two similarly-named fields. For example, [issues](issues.md), [merge requests](merge_requests.md), and [project milestones](milestones.md). The fields are:
For example, an issue might have `id: 46` and `iid: 5`.
-`id`. ID that is unique across all projects.
-`iid`. Additional, internal ID that is unique in the scope of a single project.
| Parameter | Description |
| --------- | ----------- |
| `id` | Is unique across all issues and is used for any API call |
| `iid` | Is unique only in scope of a single project. When you browse issues or merge requests with the Web UI, you see the `iid` |
> The `iid` is displayed in the web UI.
That means that if you want to get an issue via the API you should use the `id`:
If a resource has the `iid` field, it is often used instead of `id` to fetch the resource.
```
GET /projects/42/issues/:id
```
For example, suppose a project with `id: 42` has an issue with `id: 46` and `iid: 5`. In this case:
On the other hand, if you want to create a link to a web page you should use
the `iid`:
- A valid API call to retrieve the issue is `GET /projects/42/issues/5`
- An invalid API call to retrieve the issue is `GET /projects/42/issues/46`.
```
GET /projects/42/issues/:iid
```
> Not all resources with the `iid` field are fetched by `iid`.
