manufacturers assign back & forth mousebuttons in an unstandardized way. Most users will, however, want the BACK button on the more accessible / easy to reach button. To achieve this they can run like

bash -c     xinput set-button-map                   11    1 2 3 4 5 6 7 9 8

if they figured out their mouse is #11 and the buttons in question are 8 and 9 to be swapped. Would be nice to offer that option in system settings to make the mice more convenient. Not sure about the percentage of mis-assigned mice on the market though. But it is rather inconvenient if a user has 2 mice in alternating use where those BACK / FORTH buttons are incoherently located. Swapping those buttons is major usability improvement for these users.