Indices have special meaning in Tk.

INDICES
       Many of the widget commands for listboxes take one or more indices as
       arguments.  An index specifies a particular element of the listbox, in
       any of the following ways:

       number      Specifies the element as a numerical index, where 0
                   corresponds to the first element in the listbox.

       active      Indicates the element that has the location cursor.  This
                   element will be displayed as specified by -activestyle when
                   the listbox has the key‐board focus, and it is specified with
                   the activate widget command.

       anchor      Indicates the anchor point for the selection, which is set
                   with the selection anchor widget command.

       end         Indicates the end of the listbox.  For most commands this
                   refers to the last element in the listbox, but for a few
                   commands such as index and insert it refers to the element
                   just after the last one.

       @x,y        Indicates the element that covers the point in the listbox
                   window specified by x and y (in pixel coordinates).  If no
                   element covers that point, then the closest element to that
                   point is used.

       In the widget command descriptions below, arguments named index, first,
       and last always contain text indices in one of the above forms.

An argument of Scrollbar.activate() obviously is not an index. On other hand, the "element" parameter is used consistently in other methods: Spinbox.invoke() and Spinbox.selection_element().


About the set method Tk inter documentation says:

       pathName set first last
              This command is invoked by the scrollbar's associated widget to
              tell the scrollbar about the current view in the widget.  The
              command takes two arguments, each of which is a real fraction
              between 0 and 1.  The fractions describe the range of the
              document that is visible in the associated widget.  For example,
              if first is 0.2 and last is 0.4, it means that the first part of
              the document visible in the window is 20% of the way through the
              document, and the last visible part is 40% of the way through.

It would be better to use same parameter names as in Tk.

I am planning to update all Tkinter docstrings from more detailed Tk documentation in separate issue.