When you use someone else's work as your own, without giving them credit and claiming it as your own, that's plagiarism. Therefore, any modifications you make to something using someone else's work must either be private and not released to the public, or if permisson is given, to release it with giving credit where it's due.
I am familiar with the rules about credits. Thats not what I am asking about...I am asking wehter a mod is private when "you" share it with someone else?