Typically, when you press down with your pen and touch the tablet it will be interpreted is a "click" action - just as if you left clicked with a mouse.
However, in some rare circumstances and depending on the tablet and tablet brand and tablet driver, you MIGHT be able to modify the behavior to a slight degree.
Sometimes you can enable or disable the clicking of the tip
I don't know of any way to assign the tip to perform some other behavior when pressed. For example, no way for the tip to be assigned to the SHIFT key or something.
Wacom pens just always "click". There's no way to change this behavior.
The Huion driver lets you disable clicking.
In the Driver UI it is a little confusing though. The tip is shown as being in one of two states: valid and invalid.
The default is valid and this means the pen works normally.
The invalid setting, disables clicking.
In the XP-Pen driver, you can only disable the tip.
In the driver, press the gear icon and you can disable the tip by checking the box that says disable pressure.