A USB Video Class or UVC device for short is any advice which is capable of streaming a video, whether it be a webcam, digital camcorder, a video converter for analog or a simple still-image camera. So as long as the device supports video streaming, it may be called a UVC device.
Using the correct pieces of software, one is able to access the video controls of such devices, allowing you to not only change the recording specifications (resolution, quality, etc) but also the physical condition and features of the camera itself. These include the ability to zoom, pan, tilt, roll, focus, a change in exposure or even enabling scanning mode.
So long as the camera supports one of these features, a suitable piece of software should be capable of providing an easily used and understood user interface to access these controls.
Then there's the digital side of the picture, settings which can't be controlled physically. These include brightness, contrast, saturation, sharpness, hue, gamma and digital zoom but definitely don't stop there. Similar to the physical control options, support for any of these is optional since the primary function of the device itself is streaming video whereas these features fall into the category of secondary, optional features and their only benefit is adding to the functionality of the camera itself.
Absolute and Relative controls
Similar to other real life situations, the major difference between absolute and relative control in a UVC device is quite simple. Absolute controls simply change the current settings to the desired ones whereas relative controls increment or decrement values to the current settings. But this is not always the case.
Exposure time controls (Case 1)
So by passing the value 1 to Relative exposure time controls, you increment the exposure time by one step whereas in the case of Absolute exposure time controls, by passing a value of 1 you're simply setting the exposure time to 1/10,000th of a second. A value of 10,000 would set the exposure time to 1 second.
Pan & Tilt controls (Case 2)
Taking a look at pan & tilt controls. There's a difference between how absolute and relative settings operate. When it comes to Absolute, there's two variables.
One for controlling the pan settings and the other for controlling tilt settings, both in arc-second units allowing you to easily inform the camera which direction it should rotate in and at which speed.
But as far as relative controls are concerned, there's four different variables, two each for pan & tilt letting you control speed separately while the other pair of variables are only there to let the camera know when it should start moving.
Absolute or Relative?
At the end of the day, cameras are still able to offer both absolute and relative controls for various features, letting the consumer choose which option he prefers to use.
All these settings and variables are only involved in the backend, a host software is able to receive the appropriate values for each functionality that the camera is able to provide and this host software is then capable of displaying and allowing the control of these values in a user interface which anyone can use and understand without issues or any difficulty in understanding the various controls.