This is used for some client apps for graceful degrading:

A few other Subsonic server variants identify themselves to client apps by adding an extra “type” property to the response, for example:
{
  "subsonic-response": {
    “type”: “funkwhale”,
    "status": "failed",
    "version": "1.16.0",
    "error": {
      "code": 40,
      "message": "Wrong username or password."
    }
  }
}

Doing so would allow the client app to adapt more gracefully to partially implemented APIs

This is suggested by one of the developper from the play:Sub app.