> ## Documentation Index
> Fetch the complete documentation index at: https://docs.gobl.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Header

Header defines the metadata of the body.

## Schema ID

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

## Properties

| Title  | Property | Type                                       | Description                                                                                                                     |
| ------ | -------- | ------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------- |
| UUID   | `uuid`   | string                                     | Unique UUIDv1 identifier for the envelope.                                                                                      |
| Digest | `dig`    | [dsig.Digest](/draft-0/dsig/digest)        | Digest of the canonical JSON body.                                                                                              |
| Stamps | `stamps` | array of [head.Stamp](/draft-0/head/stamp) | Seals of approval from other organisations that can only be added to non-draft envelopes.                                       |
| Links  | `links`  | array of [head.Link](/draft-0/head/link)   | Links provide URLs to other resources that are related to this envelope and unlike stamps can be added even in the draft state. |
| Tags   | `tags`   | array of string                            | Set of labels that describe but have no influence on the data.                                                                  |
| Meta   | `meta`   | [cbc.Meta](/draft-0/cbc/meta)              | Additional semi-structured information about this envelope.                                                                     |
| Notes  | `notes`  | string                                     | Any information that may be relevant to other humans about this envelope                                                        |

## Validation Rules

| Field    | Test                                                                                        | Validation Code / Message                                                     |
| -------- | ------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------- |
| `uuid`   | <ul class="gobl-test"><li class="gobl-test-present">Present</li><li>Has timestamp</li></ul> | `GOBL-HEAD-HEADER-01`<br />Header must contain a UUID v1 or v7 with timestamp |
| `dig`    | <ul class="gobl-test"><li class="gobl-test-present">Present</li></ul>                       | `GOBL-HEAD-HEADER-02`<br />Header must have a digest                          |
| `stamps` | <ul class="gobl-test"><li>No duplicate stamps</li></ul>                                     | `GOBL-HEAD-HEADER-03`<br />Duplicate stamp providers are not allowed          |
| `links`  | <ul class="gobl-test"><li>No duplicate links</li></ul>                                      | `GOBL-HEAD-HEADER-04`<br />Duplicate link keys are not allowed                |