excerpt from man imwheel

       -b, --buttons button-spec
              Remap buttons in button-spec to interpreted wheel/thumb input.  Also limits the button grab to the specified buttons when using the ZAxis method.  (see "X WINDOWS  ZAXIS  METHOD"  below)  the
              button-spec may specify any of up to five buttons.  the button-spec is decoded in the following order for wheel input:

                     Index   Interpreted As    Button Number
                     1       Wheel Up          4
                     2       Wheel Down        5
                     3       Wheel Left        6
                     4       Wheel Right       7
                     5       Thumb Button 1    8
                     6       Thumb Button 2    9

              A button-spec of "45" will limit the grabbed buttons for only wheel up and down.
              A button-spec of "67" may be useful to use actual buttons 6 and 7 as wheel up and down, and limit the grab to only those two buttons.
              A button-spec of "0" turns off any defined mapping, thus allowing for skips in the button-spec for something that doesn't exist on your mouse.
              A button-spec of "45006" may be for normal wheel up/down and a thumb button 1, but no horizontal wheel axis.
              The default button-spec is "456789".
              See also xmodmap(1).