The second radio system on an FPV multi-rotor is, of course, the control system. Again, both a transmitter and receiver are needed, and the choice of frequency is an important one. There are generally fewer frequencies available for control systems. Most common is 2.4 GHz, though 35 MHz and 72 MHz were popular in the past, also. UHF systems are becoming increasingly prevalent, and we'll discuss those in more detail, too.
Transmitters are available either as a single unit or, less commonly, as modular pieces. When you buy the transmitter shell, sticks, knobs, and switches on their own, without radio hardware, you then need a transmitter module to emit your inputs. Enthusiasts with multiple models communicating over different frequencies often find this approach useful, since it's easy to swap out modules. The Turnigy 9XR is an example of a quality, inexpensive modular transmitter.
Deciding on a frequency for control follows the same principles as video. You get better penetration from lower frequencies, and again, the same wavelength cannot be used for FPV and control. Additionally, frequencies with matching harmonics are discouraged because you'll still get interference. A 1.2 GHz video signal will conflict with 2.4 GHz control for example, since the radio waves share nodes.
As you shop around for transmitters, pay particular attention to the number of channels they offer. For each remote-controllable action, a unique channel is needed to convey input. The minimum required to pilot a multi-rotor is four channels: throttle, yaw (rotation), pitch, and roll. For every flight mode switch, gimbal control, or lighting control, an extra channel is involved. Most flight controllers are going to recommend eight channels; the more you support, though, the greater your opportunities for customization and upgrades.
On the other end of the control system is a receiver with a corresponding frequency. Of course, the number of receiver channels has to match the transmitter if you want to utilize all of the available functions. Each channel has a pinout that is connected using a servo cable to the appropriate component. The four stick outputs, at least, must be fed to the flight controller in order to control a multi-rotor.
There are two protocols for control transmission. The first and most traditional is pulse-code modulation (PCM), a standard analog one-to-one broadcast. It remains both reliable and popular. But increasingly, hobbyists are adopting an alternative: pulse-position modulation (PPM). With PPM, multiple inputs are encoded and transmitted using a single channel. It is advantageous in that it reduces wiring and setup difficulty, allowing for more channels than previously possible. Both schemes work, and neither is deemed more correct than the other. However, not all transmitter/receiver combinations support PPM.
The newer UHF solutions mentioned previously are fundamentally similar to other control systems. However, they operate across a range of frequencies (usually 130 to 135 MHz) and use channel hopping to maintain a strong link for as long as possible. Generally, UHF transmitters are housed in external cases attached to a conventional transmitter using a trainer port. At such low frequencies and relatively high power, UHF systems are considered standard for long-range applications, with connections reaching out several miles and passing through obstacles with comparative ease.
There are various brands of UHF equipment, such as DragonLink and ImmersionRC EzUHF. All facilitate the same functionality, though some are more compatible with certain transmitters. You can expect to pay more for a UHF-based setup, and it might not even add anything to your FPV experience. Enabling such long range is unnecessary for anyone piloting small quads or flying in more confined spaces. Moreover, lower-power transmitters are preferable to maintain transmission balance and prevent conflict.