You would use this where you know you will have too many pages for a notebook widget.
The groupbox first appeared in TclMacBag v0.09.
For a menu-less "groupbox" which simply acts as a container, see the boxframe
set gb [::tclmacbag::groupbox .g]
$gb add [ttk::frame $gb.a1] -text "Cats"
$gb add [ttk::frame $gb.a2] -text "Monkeys"
$gb add [ttk::frame $gb.a3] -text "People"
pack $gb -expand yes -fill both -padx 10 -pady 5
An opinion: This is a wonky looking widget that probably shouldn't appear in real programs.
Its aim is to provide a notebook which can display many tabs without resorting to those horrid multi-line tabs Microsoft inflicted upon us with Windows 95.
However, a more modern approach would be to use a listbox/treeview showing the options and change tab/frame contents according to which one is selected.
(See Figure 6-25 of Gnome's Human
Interface Guide for an example.)
Always control your toplevel's dimensions when using this widget (using either [wm geometry] and/or [wm minsize]) to ensure frame contents (including buttons) are visible.