The opacity tab is for transparency settings. Setting Opacity to 1.00 makes it opaque while 0 makes it entirely clear. Menus, inactive windows, active windows titlebars and borders all can have their opacity changed indepently.
Fading windows can have some differences. There is a checkbox for fading window during opactity changes so if the opacity changes it will smoothly fade. With this settings windows will fade in and out when opened and closed unless you don't them to which you can press the Avoid fade in out when windows opening and closing.
from the command line. From LXQt configuration center press the gear button with a c in it to launch Window Effects. Of course for this to have any effect the compton compistor needs to be running so see chapter 3.2.13 for how to autostart applications.
