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I posted one ready-to-use answer that biols down to this:
the XOOPS project
interpretation of answering the question whether a XOOPS module is a derrived product of the GPL source code, is based on the level of integration with the XOOPS core. IF your module depends on using the XOOPS core (classes) (or any other GPL code, for that matter) for it's
primary functionality to work (database connestion, data handling, permission management), then it is considered to be
part of the whole program (as it can't function as intended without it), and therefore gets the hereditary GPL license. If the module can function as intended (perhaps not fully, but in it's primary functions it can), and can run as a standalone script too, then it can be licensed as you see fit.
This is not
just an opinion, this is the result of many discussions with legal experts, other open source projects and members of the community.
I hope that helps
Herko