Use checkboxes to search for records with a given properta. If only records with this property are to be searched for, the state of the box should be  selected, if these records are to be omitted from the search, the state should be  deselected. If the state is  undefined, then both records with and without that property are searched for. Examples use the module Multi-day and Recurring Events^M

1. By default, checkboxes are usually  undefined. The values of the field are not taken into account for the search.
   Example:  Multi-day event finds both one-day and multi-day events.
2. If the box is  selected with a single left-click, only records that have that property are searched for.
   Example:  Multi-day event only finds multi-day events.
3. If the box is  deselected by left-clicking again, then records with the selected property are excluded from the search results.
   Example:  Multi-day event will not find multi-day events.
4. A third left click restores the status  undefined .