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org.apache.struts2.interceptor.CheckboxInterceptor is in the defaultStack. It checks each form parameter submitted to the action and if it finds one with a prefix of _checkbox it inserts a value for a parameter whose name is derived from the suffix to _checkbox if it does not exist. The default value inserted is false but this can be changed by setting the uncheckedValue parameter on the interceptor.
This means that a checkbox can be accompanied by a hidden input with the same name but a prefix of _checkbox so that if the checkbox is not checked on the form the action will still receive a value rather than the default HTML action of not providing a value for unchecked checkboxes.