AOVs are set up by using a combination of ArnoldOutputChannelDefine and RenderOutputDefine node pairs, where you set up the AOV in the former, creating a channel, and then the latter use that channel to set up an actual render output. Once they are defined, there is no need to connect them, they are just available and if you create a Render node you will see them listed as outputs.
Can be used to add global AOV shaders like cryptomatte.
This is a placeholder parameter for output channel definitions. If you select the root location, go to its attributes, and expand the channel definitions and
outputChannels parameter, you will see any channels you have defined via
ArnoldOuputChannelDefine nodes. For example: