A "beat reporter" is a journalist specialized on a subject but he does not report only from one place or source organization or political institution as the "setorista" does. Also, a "setorista" has to be accreditated by the institution he covers. For instance, Mr. xxxx, a White House correspondent...
In Portuguese, we don't say "setorista" for war correspondents but English has just one word for both the journalists reporting from specific places and organizations.