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Signatures and marketing campaigns store their visual layout as a content block tree: a nested JSON structure of typed blocks. When you create or update a signature, you send this tree as draft_content. For a campaign, you send it as content.
The full schema is documented as signature_content on the Create a signature (POST /v1/signatures) request in the endpoint reference. This page explains how the tree is shaped so your writes pass validation.

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The shape of the tree

The tree always starts with a single stack block at the root. Every block has a type, and a stack holds an ordered list of child blocks in its children array. A child can itself be a stack, which is how rows, columns, and groups are built. 
{
  "type": "stack",
  "children": [
    { "type": "text", "...": "..." },
    { "type": "image", "...": "..." }
  ]
}

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Block types

TypeRenders
stackA container that lays out its children as rows, columns, or groups.
textA line of text, built from text tokens and smart-field tokens.
imageAn image, such as a logo or a photo.
iconA single icon.
buttonA call-to-action button with a link.
listA labelled list, for example a block of contact details.
socialListA row of social media icons.
dividerA horizontal separator.
marketingA promotional banner driven by a marketing campaign.
The list and socialList blocks contain their own item blocks (listItem, listItemLabel, and socialListItem).

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Smart-field tokens

Text and other blocks can include smart-field tokens that resolve to a teammate’s data, such as their name, job title, or phone number, when the signature renders. This is how one template produces a personalized signature for every teammate. Define the available fields with the smart fields endpoints (GET /v1/smart_fields and POST /v1/smart_fields).

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Write content safely

Validation is strict. An invalid tree is rejected with 422, and the response pinpoints the offending field. The safest way to produce a valid tree is to start from one that already renders:
1

Read an existing signature

Fetch a signature whose layout is close to what you want with GET /v1/signatures, and copy its content.
2

Modify the tree

Change only what you need, for example swapping an image URL or editing the text of a block, and keep the structure intact.
3

Write it back

Send the updated tree as draft_content on POST /v1/signatures or PATCH /v1/signatures/{id}.
To clear a content field, send null.

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Publish your changes

Writing draft_content updates the draft only. To make a signature live, call POST /v1/signatures/{id}/publish. Publishing copies the draft into the published content and triggers installation for the assigned recipients. To re-trigger installation without changing the content, call POST /v1/signatures/{id}/install.