The multi-select check-box WIDGET described The MULTISELECT Widget displays a column of check-boxes used to select items in a LIST. The following configuration setting causes a check-box to be displayed in the column header that can be used to select or de-select all of the check-boxes at once.
<ENABLE_SELECT_ALL_CHECKBOX>true</ENABLE_SELECT_ALL_CHECKBOX>
Please note if the ENABLE_SELECT_ALL_CHECKBOX element is not specified, it defaults to FALSE.