Treeview is a type of widget.
The treeview widget that comes with tk since 8.5 is ttk::treeview]. This treeview widget is intended to provide something that's a little more powerful (and faster) than the [BWidget::Tree widget, and a little bit simpler to use (though less powerful) than the TkTreeCtrl widget.
Sarnold uses a BWidget Tree in which some data is associated with each entry. ttk::treeview does not seem - to my great surprise - to handle other data than the text. Is it true ? Is adding such a feature planned ?