It is the total miles ridden in a year, not just those completed within an IBA certificate ride.
The ride's rules page doesn't provide much information, but what you would need to do is maintain a log and every receipt related to the years' riding - not ony gas receipts, but those for tires, tuneups, maintenance, etc. Remember - it's up to you to demonstrate you rode the ride you claim by submitting sufficient documentation to do so. You can certainly submit for other IBA certificate rides within that year-long ride.
Some examples of the reasons why some have completed the ride, I think it was Phil Mann who racked up 113,000 miles in six months when he held a job transporting motorcycle parts between Chicago and the West Coast (he won the BMW MOA mileage contest that year. I'm not sure if that record has ever been broken). Joe Mandeville rode 103,000 miles in six month while training for the Iron Butt Rally. And I think Bob Higdon made the club when he was collecting courthouses. The point is that there are lots of ways to get it done for those who really, really want it. And you really, really have to want it.
Ira Agins
Iron Butt Association