An Idea Exchange is a specialized type of message board within the overall community, one that has somewhat different posting and organizational procedures than a regular message board. The terminology is part of the jargon associated with the software system, Lithium, that drives the community.
If you look at the entire list of boards (by clicking the Browse Book Clubs link) you'll see that some are identified with a little book in a green oval icon. Those are the regular message boards. If you hover on one of those icons a tooltip will pop up saying "Message Board." Other boards have a different icon, which as far as I can tell represents a laurel branch. If you hover over those the tooltip says "Message Exchange." Those are the ones that obey the rules explained in the post you referred to. It looks as if there are seven of them at the moment.
The discrepancy between the terms "Message Exchange" and "Idea Exchange" is unimportant. They're the same thing. (If you really want to know, "Idea Exchange" is the official Lithium terminology while "Message Exchange" is the term in this community. Don't ask me why.)
