"Checklist Content YAML" custom fields accepts and stores data in YAML format. It contains "items" array and each entry contains obligatory "text" property (checklist item text) and optional "checked" property (boolean value for state of the checkbox). It might contain also "description" property for multiline description and "status" property if statuses are enabled. 

Here is a valid example:

items:
  - text: '--- Header (title)'
    checked: false
  - text: 'Item #1'
    checked: true
  - text: 'Item #2'
    checked: false
  - text: 'Item #3'
    checked: false

And another example with enabled statuses:

items:
  - text: '---UI tested on'
    checked: false
    status: open
  - text: 'Chrome :santa'
    checked: false
    status: in progress
  - text: Firefox
    checked: true
    status: done
  - text: Edge
    checked: true
    status: skipped
  - text: Safari
    checked: true
    status: skipped