Licensing options for WordPress Plugins

Question

Question Edited because it's off-topic (but the confusion about two statements in the original question is still valid, IMO)

====== Edited Question ===========

I'm developing a plugin for WordPress named my-custom-plugin and here's the my-custom-plugin.php

<?php
/*
 * Plugin Name: My Custom Plugin
 * Plugin URI: 
 * Description: 
 * Author: 
 * Author URI: 
 * Version: 
 * Requires at least: 
 * Tested up to: 
 * Copyright: 
 * License: 
 */

What are the licensing options that are available for the License: option above?

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====== Original Question which is off-topic =========

Isn't Themeforest (ThemeForest.net) violating the WordPress GPL v2 License?Their WordPress Products’ source codes aren’t publicly available.

Also I cannot request for a free copy of the source code of any WordPress Product they sell. Which means a clear violation of the GPL v2 license, isn't it?

Also found these confusing licensing statements at wordpress.org

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1. Statement for Themes (source: https://developer.wordpress.org/themes/getting-started/wordpress-licensing-the-gpl) : "To develop WordPress themes for the public—either free or paid— you need to get acquainted with the GNU General Public License (GPL) that WordPress uses."

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2. Statement for Plugins (source: https://developer.wordpress.org/plugins/the-basics/including-a-software-license): "Most WordPress plugins are released under the GPL, which is the same license that WordPress itself uses. However, there are other options available. It is always best to clearly indicate the license your plugin uses."

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The confusing part for me here is, Wordpress has made the theme licensing GPL but for plugins they say "However, there are other options available.". What are these other options? (Does that mean only a part of WordPress source code is GPL?)

====== Original Question Ends ==================

Answer 1

This is a legal and business question and not a technical one, but since you found an angle that is slightly relevant to plugin development, here comes an answer.

Copyright law is complex and while in many places it is basically the same, there are still differences between different countries and therefor there is no "right answer" and you will need to consult a lawyer to get an advice on what license your local law allows you to use.

There are two major stumbling block for a copyright discussion in the context of wordpress.

1. "Wordpress" do not own the copyright to its code. Unlike other open source projects, there is no official transfer of copyright from the contributor to "wordpress", and therefor the wordpress code is a patchwork of different copyright holders. The practical implication is that "wordpress" can not sue anyone to enforce its copyrights as only the actual copyright holder can sue, which means that "wordpress" is lacking the legal tools to enforce its copyright.

2. It is unlikely that you own the copyright to your code, or at least that you can just set whatever license you want. A practical example is that code on stackexchange which was published before 2016 requires attribution, which is a basic incompatibility with both GPL and BSD type licenses, therefor you can not legally declare that your code is GPL if you used a snippet from this site in it.

Assuming for a moment that the stumbling blocks do not exists, the question is whether the old assertion by @matt that by using the wordpress API you are bound by the GPL, for which the US court system gave a kind of answer in the oracle vs google case by asserting that the java API is not copyrightable. While this is not the end of the discussion regarding the wordpress copyrights, it is right now a precedent against that assertion.

So what license can you use? Well, as said above it is unlikely you actually hold the full copyright and can set any license at all, but if you want it to be part of the wordpress repositories you will have to pretend as if you can do it, and set the license to something which is GPL compatible.