Data definition:GtkListStore

["William Walter" <wwalterw gmail com>]

May be i'm overlooking the obvious explanation for this but I can't find the answer.
Gtk+ source code folder(../gtk+-2.8.10/demos/gtk-demo) has a demo program 
editable_cells.c. It displays total number of products, product name and taste level
(yummy) in a treeview widget. So the program uses GtkCellRendererCombo for the
first column, GtkCellRendererText for second and GtkCellRendererCombo for the 
third . It declares the list store(model) for the treeview widget and add the data 
to the model like this:

.............
enum
{
  COLUMN_ITEM_NUMBER,
  COLUMN_ITEM_PRODUCT,
  COLUMN_ITEM_YUMMY,
  NUM_ITEM_COLUMNS
};


static GArray *articles = NULL;

static void
add_items (void)
{
  Item foo;

  g_return_if_fail (articles != NULL);

  foo.number = 3;
  foo.product = g_strdup ("bottles of coke");
  foo.yummy = 20;
  g_array_append_vals (articles, &foo, 1);

  foo.number = 5;
  foo.product = g_strdup ("packages of noodles");
  foo.yummy = 50;
  g_array_append_vals (articles, &foo, 1);

  foo.number = 2;
  foo.product = g_strdup ("packages of chocolate chip cookies");
  foo.yummy = 90;
  g_array_append_vals (articles, &foo, 1);

  foo.number = 1;
  foo.product = g_strdup ("can vanilla ice cream");
  foo.yummy = 60;
  g_array_append_vals (articles, &foo, 1);

  foo.number = 6;
  foo.product = g_strdup ("eggs");
  foo.yummy = 10;
  g_array_append_vals (articles, &foo, 1);
}

static GtkTreeModel *
create_items_model (void)
{
  gint i = 0;
  GtkListStore *model;
  GtkTreeIter iter;

  /* create array */
  articles = g_array_sized_new (FALSE, FALSE, sizeof (Item), 1);

  add_items ();

  /* create list store */
  model = gtk_list_store_new (NUM_ITEM_COLUMNS, G_TYPE_INT, G_TYPE_STRING,
        G_TYPE_INT, G_TYPE_BOOLEAN); //<--why G_TYPE_BOOLEAN is used here?

  /* add items */

  for (i = 0; i < articles->len; i++)
    {
      gtk_list_store_append (model, &iter);

      gtk_list_store_set (model, &iter,
                          COLUMN_ITEM_NUMBER,
                          g_array_index (articles, Item, i).number,
                          COLUMN_ITEM_PRODUCT,
                          g_array_index (articles, Item, i).product,
                          COLUMN_ITEM_YUMMY,
                          g_array_index (articles, Item, i).yummy,
                          -1);
    }

  return GTK_TREE_MODEL (model);
}

.....


what's the use of fourth column(G_TYPE_BOOLEAN) used during the definition of
treeview model? You can see that no data is associated with this column. so why
use it?