Hi,
you get a list of what Unity sees as web cameras being registered by the Operating system.
so if you only see a Rear cam, it's your OS/Unity that only sees this one. if you plug a Usb cam, make sure you refresh, I know on windows for example, plugin a joystick while the app is running won't register it, you'll have to restart. it may be the same for webcams.
Bye,
Jean