I want to write a few modules for ERP system, which is licensed under GNU GPLv3, and I want to sell them. However I don't want my users to be able to redistribute the code without my permissions. Considering the fact that I won't touch the code of the ERP system, but I would use it's API, could I do this? I've read a few similar questions here. However I wasn't able to understand am I allowed to do it. Here is a quote from this question:
Case 1: If you
use a library X that comes under GPLv3 or AGPLv3, use it as a library (that is, keeping it as a separate entity without copying source code from the library into your own source code), and do not modify it,
you do not have to make available anything.
But I can't understand am I using the ERP as a library or not