My 6-button mouse

[richmond62]

Both buttons 4 and 5 provide the same service inwith a web-browser:

On clicking button 4 or 5 an odd, grey 'thing' appears in the browser window:
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Screenshot 2020-02-24 at 12.44.15.png (12.58 KiB) Viewed 1836 times

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Once that symbol appears one can navigate up and down a browser window by, simply, pushing the mouse
forwards or backwards.

Clicking button 4 or 5 for a second time returns the mouse to standard functionality.

[LCMark]

@richmond62: It says precisely what the buttons on the device do in the technical spec sheet:

6 Buttons (Left & Right Click, Back & Forward,
DPI Switch, Scroll Wheel and Middle Button)

One is to control sensitivity of device (DPI Switch) - so that's not an input button

Three are the 'normal' mouse buttons (Left, Middle, Right)

And two are 'Back and Forward' buttons - doing a google search for - "back and forward mouse buttons mac os x" should give you the info you need to leverage them in macOS apps generally.