WYSIWYG

The WYSIWYG field can be used when you want to allow user to use the visual editor instead of the raw text input.

The syntax is very similiar to the textarea field, the main difference is a lack of support for the rows and cols parameters:

{
    "name": "footerText",
    "label": "Footer text",
    "group": "Footer",
    "value": "",
    "type": "wysiwyg"
}

Please remember that you can define separate custom formats which will be available only in the theme settings WYSIWYG editor. They are described in the Post Editor API.