The boolean data type

Scalar data type. true or false.

...
<checkbox id="chkbox">
  <validation>
    <validator cond='{chkbox.checked == false}'
      message="The 'checked' boolean is now true."/>
    <validator cond='{chkbox.checked == true}'
      message="The 'checked' boolean is now false."/>
  </validation>
</checkbox>
...
The validation messages for this checkbox are toggled by the boolean value of its checked property

Figure 267. The validation messages for this checkbox are toggled by the boolean value of its checked property