Do you remember small icons with menu actions usually shown at the top left or top right corner? Over the years, they become retired in some desktop environments, evolved into app indicators, or be left untouched. Nuvola supports app indicators (Unity, GNOME, elementaryOS), notifier items (KDE), and legacy Xorg icons.
To enable Tray icons, open Preferences dialog from the Menu button, toggle Tray icons on and then click the gear button to adjust Tray icons options.
There two technologies to show "a small icon with menu actions":
GNOME is the default desktop environment of Ubuntu 17.10 and later, Fedora and many other distributions. Learn more about Nuvola's integration features for GNOME.
There is a difference between the original GNOME and tweaked GNOME in Ubuntu.
Unity is the default desktop environment of Ubuntu 17.04 and older. It was replaced with GNOME since Ubuntu 17.10. Learn more about Nuvola's integration features for Unity.
Unity supports app indicators out of the box. In fact, they were originally created for the Unity desktop.
The elementaryOS's desktop environment is called Pantheon Learn more about Nuvola's integration features for elementaryOS.
ElementaryOS adopted app indicators from Unity.