What is the extra fields “Default” setting for?
The extra field “Default” setting can be used to set the default(preselected option) for these field types:
checkbox, checkbox-multiple, radio, select, or select-multiple.
Like in this example for a select:
To make “Green” the default selection: set “Default” to 2.
You have a checkbox to “Subscribe to email newsletter” and you want it selected by default.
Set “Default” 1