The User option is taken into account only when Abyss Web Server is run as root. In such a case, the web server changes its executing user to what is specified in the Operating System User parameter immediately after Abyss Web Server is launched and after each restart of the server. The executing group is also changed to the value set in the Operating System Group parameter.
The server cannot be run as root for security considerations. But you can override this limitation adding the -r switch on the command line when launching Abyss Web Server.
If you run Abyss Web Server as a normal user (not root), it ignores the Operating System User and Operating System Group parameters and continues its execution without changing the user and the group.