In version 3.5.0 ‘shadow session’ referred to both sharing the physical desktop of another user, and sharing the X11 desktop. In version 4 it’s the same concept, but you don’t configure the GUI to run a ‘Shadow’ session. Now most of the session configuration happens at runtime based on information provided by the server after authentication to the server has taken place. This includes the type of session (shadowing of the physical screen or creation of a new virtual desktop), the X client or desktop environment (GNOME, KDE, custom session, etc.), the display resolution, the configured printers and so on.
The free version allows one connection to the desktop. So anyone connecting to the computer from another device will count as one connection. To allow others to connect too, you will need Enterprise Desktop.