All the options on the Video tab under RV Preferences are per display device, a.k.a. Output Device. As you switch through the Output Device choices you can individually set all of the listed settings for each device.
The most important option is “Use as Presentation Device.” Only one of the devices can have this distinction and you need to have more than one display to have any Presentation devices, since you will need a control screen as well. Therefore if you have only one item in the Output Device list Presentation mode cannot be used.