Long story short, they are URLs that are invokable as "shell commands" in Windows. As long as LIV is installed, and has been run once on a system, these commands will work when called from anywhere. Some commands will automatically start LIV, and boot LIV Capture. Some commands won't do anything if LIV is not already running and active.

You can call them manually, using the Windows "Run" prompt (Win+R).
You may also create shortcuts to these commands using Windows Explorer: New -> Shortcut, then enter your command. You can then create a hotkey for that shortcut by modifying its properties.

Behind the scenes, LIV registers itself as a URI Scheme handler for the liv-app "protocol". When Windows is asked to execute a URI as a shell command, it will look for a registered handler, and then start that program. When LIV is already running, a second instance of LIV is loaded - it then sends the command to the first instance of LIV before closing itself.
Naturally, this process takes some time. We don't recommend using this interface for high-frequency control of LIV.
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These commands interact with LIV Capture for PCVR. These commands will not automatically launch LIV Capture if LIV is not already running.

liv-app://camera/set/<INDEX>

EG. liv-app://camera/set/2

Changes the currently active camera in LIV. The index is the number of the camera in the camera list, starting from 0. If you provide a camera index that is "out of bounds", the command will be ignored.
You may re-order cameras in LIV by modifying the configuration file in %localappdata%\LIV\App - modify the CompositorBasic.CameraProfiles array.

liv-app://camera/set/fovOverride/<NUMBER>

EG. liv-app://camera/set/fovOverride/43.5

Sets the current camera's FoV override value.
Set the FoV to 0 to clear the override.
Has no effect on avatar cameras; see avatar camera plugins.

liv-app://camera/add/fovOverride/<NUMBER>

EG. liv-app://camera/add/fovOverride/-1.5

Adds to the current camera's FoV override value.
If no override is set, it will use the camera's native FoV, then add to it as the override.
Has no effect on avatar cameras; see avatar camera plugins.

liv-app://camera/toggle/freeze

If you are using LIV with a tracked camera (IE. You have a SteamVR tracker attached to a camera) and you are not moving it, you can temporarily freeze it in place.
This command toggles the camera's frozen state.

All of the below commands will launch LIV if it is not running, then execute the command.

liv-app://launch/compositor

Starts PCVR Capture if not already running.

liv-app://launch/streamerkit

Starts LIV StreamerKit.

liv-app://focus/app

Brings LIV into the foreground.

liv-app://capture/manual/pid/<PROCESS ID>

EG. liv-app://capture/manual/pid/1337

Starts PCVR Capture if not already running, then switches to manual capture mode and attempts to capture the process ID given.

liv-app://launch/steam/<STEAM APPID>

EG. liv-app://launch/steam/1098100

Starts PCVR Capture if not already running, then switches to auto capture mode and launches the Steam game provided. Will not launch Steam games that are not detected by LIV. See the command line documentation for how to get a list of installed LIV games.
Once the game is loaded, LIV will automatically begin capturing it.

liv-app://launch/viveport/<VIVEPORT GAME ID>

EG. liv-app://launch/viveport/8fd7deb0-64d8-48b1-bd44-87db84f37de8

Starts PCVR Capture if not already running, then switches to auto capture mode and launches the Viveport game provided. Will not launch Viveport games that are not detected by LIV. See the command line documentation for how to get a list of installed LIV games.
Once the game is loaded, LIV will automatically begin capturing it.