I can only add that the act doesn't provide for a particular process that depends on the nature of a request or a document that has been requested. It's really up to the ATIP office to decide what sort of consultation they need. Sometimes it's with another department that has an interest in the issue, or with the Treasury Board or the Department of Justice. It's up to them to decide if they need assistance in dealing with that request.
There's no process in place, except that the services are available and the people who do the work are aware of their existence.