We do have a definition specifically in the agreement. We actually stipulate that they cannot work.... Nothing they do is mandatory. They come in as they please. They cannot be disciplined if they only do three hours, for instance.
In my volunteer experience, I was coming in as part of a class from the University of Ottawa. I was only doing three hours a week. It was on my own schedule. I was not doing work that would replace the work of a bargaining unit member, so I wasn't replacing the parliamentary staff in their tasks.