Steps to reproduce:
1. Go to XFCE Settings Manager/Window Manager Tweaks/Accessibility
2. Set "key used to grab and move windows" to "Alt"
3. Try to drag windows with the right ALT key, as well as the left
The user can move windows around with either ALT key while clicking and dragging.
The user can only move windows with the left ALT key while clicking and dragging. This breaks my personal workflow, as I use the mouse on this laptop exclusively with my right hand, and can easily move windows around in GNOME by pressing ALT with my right thumb.
Xfce does not specify neither left nor right Alt, it just use "Alt".
But you can use any other other predefined modifiers (Mod<n>) but if it's not listed chances is that it's not a modifier.