AvdeviceRegisters FFmpeg's capture and playback devices with libavformat, for the whole process. Loading this module already does it: the call exists so that a program can name something from this library and make the linker keep it.
Once registered, the device formats are among the formats Av.Format.find_input_format and Av.Format.guess_output_format find. A device is opened as a container with Av.open_input or Av.open_output, the device as the URL, or with Av.open_output_format, and takes its options through ?opts of those functions. Which devices exist depends on the FFmpeg version, build and platform.
It raises nothing, holds the runtime lock, and may be called from any thread at any time.