AudioLibrary.OnUpdate object contains undocumented value

receiving this notification:

Code:
{
  "jsonrpc": "2.0",
  "method": "AudioLibrary.OnUpdate",
  "params": {
    "data": {
      "id": 491,
      "type": "album"
    },
    "sender": "xbmc"
  }
}

you will find type “album” is not in the api spec.
It only contains one enumerated value for Notifications.Library.Audio.Type

Code:
"type": {
    "enum": [
        "song"
        ],
    "id": "Notifications.Library.Audio.Type",
    "required": true,
    "type": "string"
}

this leaves me wondering how many other types there are….