Checkbox fields are useful when an information on the card is a choice between Yes or No. A _Checkbox_ is created. A default choice defines if the Checkbox is checked or not, and the user can change it to modify the value on the card.

"fields": [
    {
        "name": "trueornot", "label": "True ?",  "type" : "checkbox", "default": true }
]

To set a field to the Checkbox type, set the type property to checkbox. The default property can then be set to true or false (and not anymore to a text value).

The Checkbox generates a Boolean Variable.