Configuring behaviour

Show/Hide “All” item

By default, a item called “(All)” is added at the top, to allow to select/unselect all items at once.

You may show or hide this item from the Show (All) Items check box under Basic Settings :

Send Value on Select All

By default, the Check box widget will send the Keyword “All” in the parameter without differentiating every element if all elements are selected.
But in the case of passing parameter to a Stored Procedure or Displaying them in a label, It could be more revelant to list all items instead of keyword “All”
If the checkbox Send Values on Select All is checked, it will work as suggested and all elements will be listied in the parameter.