Suppose you've seen the phrase, International Residence Certificate, but don't know exactly what it is. In that case, Interval International offers these "bonus" weeks to people who own high value or high demand timeshare weeks. In a system where one week is directly exchanged for another, you can see why owners of high-value weeks might not even contribute theirs. Thus, Interval International offers those extra weeks as an incentive to contribute those weeks to the exchange pool. However, your exchange options with a certificate of occupancy will be limited and are essentially leftovers of what other people have not taken in 59 days from the date of registration.