These options let you override the command that runs when a folder in the Folder Tree is selected using the mouse. You can configure commands for both the left and middle mouse buttons (the right mouse button is always used for the context menu), and you can have separate commands for when the Alt, Control and Shift keys are held down.
The command for each event must be a command that would make sense on a button or hotkey. You can use Opus internal commands or external programs (passing arguments to them using the standard control codes). If you want to run more than a simple single-line command, you can create a more complicated User command and then specify the name of the user command for the event.
The default settings for these events are:
So from the above we can see that by default, left-clicking a folder will read the folder as normal, Alt-clicking a folder will read it in a new tab, and Ctrl-clicking will read it into the dual display. Middle-clicking a folder will read it in a new tab (the same as Alt-left-click) and Ctrl-middle-click will open it in a new tab in the other display.
Say you wanted it so that Shift-clicking a folder would open it in a new Lister automatically. You could do this by setting the Left mouse button / Shift event to the command Go NEW.