@Fizzlemizz and @LanceDH thanks for the answers. It's much clearer now but it leads to another question. In the expression:
what exactly does the table "core" contains? I don't understand how a lua file can pass anything outside. Do we create that table at the end of a file? If I use a function analogy, we can return anything if we use "return" but with file, it's confusing. Thanks.