Anderton makes a good point. So, let's say, after you've subscribed to Sonar for 2 years, or that you've paid for the software by 120% using the subscription price, that version of Sonar is free to use after that, along with its updates. In other words, because you bought it using the subscription price, you pay a little extra (like Interest) until you've paid 100% for the license, and 20% for the Interest, then you own the license forever. However, to get the next version, you need to re-subscribe, or buy it outright.
Waves does this with their WUP. You can own the license of the Waves plug-ins, but if you want to get the latest versions (and in many cases, free plug-ins, if you own the larger Bundles) in the future, you need to buy WUP. Although WUP is an annual or bi-annual cost, you still get all the free updates or versions. Believe me, I used to despise WUP. But after Waves capped the price and put incentives out there for early renewal, not to mention hosting massive plug-in sales, I'm ok with it now. Their support is pretty great, too.