See discussion on Confluence Question. After removing KnownControllers in 1.5, need standardized way to indicate that a Device is a Controller. Confuence Question answer suggested to add that to the ICS documents but since this is something generic for all ICS, it better moves to the spec which is the first place to search.
Table A.67: Device Classes
New in JDF 1.6
|A Controller is a Device that is a proxy for one or more individual devices or machines.|