If you are a programmer who is interested in WordPress and designing, then there is a possibility that you have seen that WordPress is working under the terms of GPL or GNU General Public License. This GPL is quite different from the normal EULA or End User License Agreement that you often find at the end of a software installation or at the beginning. So, what is GPL and how is it different from EULA.
GPL is an open-source license which means it is free. However, as per GPL, it is not free as in terms of price but it is free as in the term ‘free’dom. If you are using any program under the GPL, you can use it to modify or distribute any software of your need. GPL will not limit your freedom to distribute. It is here that GPL becomes different from EULA. Under EULA, you are given a non-exclusive license which means that you can only use one version of the software and you cannot modify or distribute it.
If the original software has a license under GPL, that means any modifications or modules used have to be licensed under GPL too. Plugins used in WordPress must be under the GPL license as well. If you release a theme or plugin with the GPL license then a buyer can modify it as the GPL license allows such a modification to take place. However, GPL will only come into effect if you choose to distribute a product. GPL will not be activated if you use plugins on your own website without trying to distribute it. Regarding copyright, if you attach the appropriate copyright notice with your software, the buyer can tweak your product but not claim it as their own.
So, to be on the safer side, it is best to use a split model. In a split model, you release a specific code and protect it with the GPL license but you do not include the design elements from HTML or JS. That way, you cannot be held liable. However, there are certain stores that are trying for GPL-based themes.