Logitech MX Master Frequently Asked Questions Manual page 45

By default, the Mode shift button lets you switch between SmartShift modes. When SmartShift is
enabled, you can use the scroll wheel in either ratchet mode or hyper-fast spin mode. See Adjusting
your mouse movements for more information. You can also assign a different action to the Mode shift
button if you wish.
NOTE: Click More and then scroll down to see the full list of available settings.
Speed-adaptive control wheel (Middle button)
In addition to using the control wheel for scrolling, you can also assign functions to it, just as with any
other button. For example, if you assign the action "Close window", when you press the scroll wheel,
the current window will close. The default setting for the control wheel is Middle button.
To fine-tune how your scroll wheel works and feels, see Adjusting your mouse movements.