You are correct. Clydesdale was an attempt to recognize that it takes a lot more to move a larger body over the same distance, and so they gave us our own category. It used to be any male over 200 lbs was a Clydesdale. However, starting with this 2013 season USAT has changed the official weight limit to 220 pounds.
What it means for you as far as races go is that IF you are doing a race that has that division you can register for it. Supposedly it levels the playing field more than racing in your age group. Theoretically, you have a better chance of being on the podium.
I'm new but wiki says : under USA Triathlon rules, a "Clydesdale" athlete are those men over 220 pounds, while "Athena" athletes are women over 165 pounds. Other races and organizations can choose whether or not to offer Clydesdale- and Athena-type divisions and set their own weight standards.
I also have read that 200 seems to be a common cut off. Hope that helps.