insert a non-database field on server side datatable

here i have some code

/* Array of database columns which should be read and sent back to DataTables. Use a space where
 * you want to insert a non-database field (for example a counter or static image)
 */
    $aColumns = array( 'engine', 'browser', 'platform', 'version', 'grade' );
/* Indexed column (used for fast and accurate table cardinality) */
$sIndexColumn = "nomorid";

now how to insert non database field in array? what is the mean of "use space" ?
sorry for basic question and thanks in advance