GPL – THE GNU GENERAL PUBLIC LICENSE “THE GNU GENERAL PUBLIC LICENSE (GNU GPL OR GPL) IS A WIDELY-USED FREE SOFTWARE LICENSE, WHICH GUARANTEES END USERS THE FREEDOM TO RUN, STUDY, SHARE AND MODIFY THE SOFTWARE.
is a free software structure which can be used, modified and distributed without any legal restrictions or limitations.
This means that once a plugin is purchased directly from the official site of its creators it can be sold, given away or used on as many sites as desired, as long as the source code remains available to its users.
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