Thanks for the reminder, fbianconi
Here's a more complete explanation.
1 Autostart
Configured by *.desktop files in directories /etc/xdg/autostart/, /etc/xfce/xdg/autostart/ and ~/.config/autostart/. For correspondingly named files, values seem to be taken from files in the directory order listed with later values overriding earlier ones.
1.1 GUI configuration
By Settings → Session and Startup → Application Autostart.
This lists an item for each .desktop file in the directories listed above which is not suppressed by a "NotShowIn=XFCE;" line. Each item is listed with the Name value from the .desktop file followed by any Comment value enclosed in parentheses. Items are shown as selected by default unless there is an effective "Hidden=true;" line.
The listing order is not yet not understood except entries deriving from ~/.config/autostart/ files are listed first.
Deselecting an item creates a corresponding file in ~/.config/autostart/ with a "Hidden=true;" line. Subsequently selecting the item changes the file to "Hidden=false;".
Items deriving from ~/.config/autostart/ files can be removed in the GUI. This removes the ~/.config/autostart/ .desktop file.
1.2 Configuration by file modification
By changing files in the directories, as described above.