In theory an extension is something that extends what is already there and a module is a creation of a new element to work with what is already there. However, they are used interchangeably, meaning, this has now (OC3) been interpreted (in opencart filing system logic) in one way like extensions are all additions and modules are a section, like payments for example.
So the bottom line is you can not assume there is any difference.
You should not modify core files .. if you would like to donate a cup of coffee I will write it in a modification for you.
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