The name of the field in the listname working list that contains item IDs. This is the non-visible, unique identifier of the tree item that can be used to identify it when selected or expanded. This is only valid for unlevelled lists and will be ignored if listtype is "levelled".
$list_caption_field. The field used to store the id information is the same field used for the caption.
The name of a valid field, in single quotes. A list of fields in the listname working list can be chosen from by clicking the corresponding dropdown button in the property sheet.