[callum85]

> That's basically a subscription model, no?

Not really. A subscription model means the software stops working when you stop paying. @entrode's pro-rating idea is just more granular about how much of a rebate you get off the next version.

[wlesieutre]

It usually means that, but not necessarily. Unreal Engine 4 does a $20/month subscription where you can stop paying at any time and just keep using the version you have.

Maybe they do that because it would be impossible to enforce anything else (paying one month gets you the full source code), but it's still great that the license allows it.