Copyleft vs non-copyleft.
Copyleft is a practice that subjects the software derived from other software to the same terms of license as the original.
There are two types of copyleft software. The traditional one that covers the whole derivative work and a weak-copyleft that covers only individual files.
A famous example of the traditional one is GNU GPL. It is the first formalized copyleft-style free software license. The software licensed under it is Linux Kernel, GNU libc, GNU GCC and others.
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