This is a documentation enhancement and should be included in the SBS CLI documentation.
It's not about server behavior but plugin behavior used by a number of plugins (IRBlaster, DenonSerial, DenonAVP/AVR Control) and currently supported by iPeng, probably some other clients soon.

Notification syntax:

<playerid> getexternalvolumeinfo <taggedParameters>

This notification notifies a client that a plugin supports volume change capability for a player.
This capability will override (from a user point of view) the "digitalVolumeControl" flag that indicates whether a player's output is fixed to a certain volume (typically 100%).
This will be used by pluginjs providing external volume control, e.g. for an amplifier (IRBlaster being an example). In this scenario, the volume output of the player is typically fixed but there is still a volume control capability and the plugin control the amp's volume instead.
The purpose of this notification is to let a client know that there is a volume control capability associated to a player.

Use:

getexternalvolumeinfo

Sending a "getexternalvolumeinfo" command triggers all plugins supporting the feature to return a notification about their capabilities for each player for which they provide these capabilities.
The command may be sent to a specific player but typically a plugin will ignore this and respond to any getexternalvolumeinfo command for each player.

Returned tagged parameters:

Tag 	Description
relative 	A boolean value stating that a plugin is able to provide relative volume change capability for this player as in "mixer volume +5"
precise 	A boolean value stating that a plugin is able to provide precise volume change capability for this player as in "mixer volume 75"
plugin 		is an optional string naming the plugin providing the capability

Example:

Request: "getexternalvolumeinfo<LF>"
Response 1: "a5:41:d2:cd:cd:05 getexternalvolumeinfo relative:1 plugin:IRBlaster"
Response 2: "a5:41:d2:cd:cd:05 getexternalvolumeinfo precise:1 plugin:DenonSerial"