In keeping with my promise to stay at the basics for a while, and still trying to respond to your questions, I make this small departure to reference. A number of calls have come in on the subject of what is a reference signal, and who cares. You can declare a signal or video source as timed or not only if you compare it to a reference. Genlock requires some sort of a reference for the whole process to work. Any production or post production system needs reference signals distributed to video sources used by the switcher.
Every device has a connector or two for reference inputs.
In most cases, the device has an internal sync generator which must be genlocked to your system. There are many types of devices with many types of needs. These rules apply to PAL and NTSC and are slightly modified for any component type of system. Encoded color systems like NTSC and PAL require sync and subcarrier at a minimum for locking a color signal. There are some tricky ways to combine the requirements into one signal.