This widget simply draws its box, and possibly it's label. Putting it before some other widgets and making it big enough to surround them will let you draw a frame around them.
Fl_Box::Fl_Box(int x, int y, int w, int h, const char * = 0) Fl_Box::Fl_Box(Fl_Boxtype b, int x, int y, int w, int h, const char *)
The first constructor sets box() to FL_NO_BOX, which means it is invisible. However such widgets are useful as placeholders or Fl_Group::resizable() values. To change the box to something visible, use box(n).
The second form of the constructor sets the box to the specified box type.
Fl_Box::~Fl_Box(void)
The destructor removes the box.