Gert, a Prosim user has clarified the subscription model on the forum –

There are two possibilities:
1) If you buy a ProSim License (B737 / A320), you get the full package included with one year free updates, starting from the date you’ve bought the package. After this free year of updates, you need to buy a yearly subscription to keep your ProSim suite up-to-date.

2) Users of Prosim B737 version 1, who migrated to version 2 received free updates up to march 31, 2018, so to keep the suite up-to-date, we needed to buy the one year subscription package.

Now for both examples, once you’ve bought the update package, you will receive all updates, starting from the end of the previous subscription period until one year later. I will continue my explaination with the users who migrated to version 2, but the same principle goes for both examples above. Again, we have two possibilities:

1) So we bought a year of updates, so our first subscription period will end on march 31, 2019. Most users will then buy a new package of updates for one year to keep the ProSim suite up-to-date.

2) Now if you decide that you don’t want to buy a new package on april 1, 2019, the updates will stop and you will no longer be able to install the latest updates. If you decide to wait for example two year to buy your next update package (april 1, 2021), then you will be able to install the updates again, but you will only be able to install the updates from april 1, 2019 up to march 31, 2020. So if you want to be up-to-date again to the very latest updates, you will have to buy a second update package so you can also install all updates up to march 31, 2021. If you wait for example five years before you want to continue again with the very latest updates, you will have to buy five update packages.

The take home message is you are only eligible for updates for one year after the date of your Prosim purchase (€1500). The updates (€99) apply retroactively from the date your subscription last expired.