the subscription is only a subscription for a design time tool: CUBA studio. It has nothing to do with the software that you produce with this tool. The software you produce will work forever without having to have a license.
In fact the software you produce is a 100% regular java application that has multiple dependencies to open source libraries. One of them is CUBA platform.
When the license expires, you cannot use that design tool anymore. So, when you want to edit the software later, you have to choose another design tool. It might be a plain text editor or something like IntelliJ IDEA.
In this case, you can pretty much compare CUBA studio and IntelliJ IDEA. Both are design time tools that will have positive impact on your productivity. None of them is required for creating and editing the software.
Regarding the license: I think it cannot be used simultaneously, but it is not a license which is linked to a particular device. But i’m not sure about it…
Where it get’s interesting is the question what happens to the premium add-ons that are just java libraries. With the subscription you get access to the maven repo which contains the libraries, which are not free (although the sources are open).
But what happens when you don’t have the license anymore? You already have the libraries as part of your software deployed somewhere. But it might be, that the next time you want to build your software, you have no access to the maven repo anymore, which means that you cannot get access to the binaries anymore.
I am not actually sure what the state of this problem is. Perhaps someone from Haulmont can clarify that.