> ## Documentation Index
> Fetch the complete documentation index at: https://gobl-cli-install-gobl-dev.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Link

Link defines a link between this document and another resource.

## Schema ID

`https://gobl.org/draft-0/head/link`

## Properties

| Title | Property | Type | Description |
| - | - | - | - |
| UUID | `uuid` | string | Universally Unique Identifier. |
| Category | `category` | [cbc.Key](/draft-0/cbc/key) | Category helps classify the link according to a fixed list. This is optional but highly recommended as it helps receivers better understand the purpose of the link and potentially how it should be presented. |
| | `key` | [cbc.Key](/draft-0/cbc/key) | Key is a unique identifier for the link within the header and category if provided. |
| Code | `code` | [cbc.Code](/draft-0/cbc/code) | Code used to identify the contents of the link. |
| Title | `title` | string | Title of the resource to use when presenting to users. |
| Description | `description` | string | Description of the resource to use when presenting to users. |
| MIME Type | `mime` | string | Expected MIME type of the link's content when the content is a file. Can only be one of the allowed types defined by EN 16931-1:2017 plus XML itself. |
| Digest | `digest` | [dsig.Digest](/draft-0/dsig/digest) | Digest is used to verify the integrity of the destination document when downloaded from the URL. |
| URL | `url` | string | URL of the resource. |
| Metadata | `meta` | [cbc.Meta](/draft-0/cbc/meta) | Meta contains additional metadata about the link. |

## Category Values

| Value | Description |
| - | - |
| `format` | Alternative formats of the same document, such as PDF, HTML, or XML. |
| `portal` | Websites that provide access to alternative versions of this document or request modifications. May also allow access to other or previous business documents. |
| `request` | Documents related to requests submitted to other systems. |
| `response` | Response documents sent from third party systems often in reply to requests. |
| `agreement` | Contracts or agreements related to this document. |
| `verification` | Evidence or verification documents that can be used to validate the authenticity of this document. |
| `attachment` | General attachments for related information such as spread sheets, letters, presentations, specifications, etc. |

## Validation Rules

| Field | Test | Validation Code / Message |
| - | - | - |
| <small>Link</small> | <ul class="gobl-test"><li>No digest without MIME</li></ul> | `GOBL-HEAD-LINK-06`<br />Link digest must be nil when MIME type is not provided |
| `key` | <ul class="gobl-test"><li class="gobl-test-present">Present</li></ul> | `GOBL-HEAD-LINK-01`<br />Link key is required |
| `category` | <ul class="gobl-test"><li>Valid or empty</li></ul> | `GOBL-HEAD-LINK-02`<br />Link category is not valid |
| `url` | <ul class="gobl-test"><li class="gobl-test-present">Present</li></ul> | `GOBL-HEAD-LINK-03`<br />Link URL is required |
| `url` | <ul class="gobl-test"><li>Url</li></ul> | `GOBL-HEAD-LINK-04`<br />Link URL must be a valid URL |
| `mime` | <ul class="gobl-test"><li>Valid or empty MIME</li></ul> | `GOBL-HEAD-LINK-05`<br />Link MIME type is not valid |


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