::::::::Third-party is in relation to the topic itself; an "independent" source. This follows the plain definition of "third-party". ("No original research" is what says, essentially, that Wikipedia and its editors are not permitted as sources.) Beyond that, Wikipedia articles are supposed to be a summary of reliably published independent scholarship about the topic. This is not achieved by depending on primary materials. That is not to say such references are not helpful or even perfectly appropriate to use in some ways. As I state above, primary sources are quite good for illustrative purposes and providing context. Third-party  (independent) sources should be the main material upon which an article draws. Not only is this good sense in the context of the project and its principle rules, it is absolutely essential to fulfill the "neutral point of view" (which is the proportional representation of the body of available reliably published material).