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You may charge for XOOPS, its installation, its modifications, its configuration, its extensions, its maintenance, its ...
But all your (programmatic) works become GPL!
This means your client has to receive full source of everything and he has the right to use it (for free), to copy it, to modify it and to distribute it further to others (even for free), or not.
For specific questions and an extensive FAQ click on the FSF label, here at the right under Powered by.