A specialism just means that some journalists concentrate on a particular area of news. Some of these areas consist of Politics, education, agriculture, law/crime, fashion, sports, music, religion, health and well-being, travel, entertainment, celebrity and many others.
Some categories fall as sub- categories. A sub-category is a branch category off a larger main category these than to focus in on key areas of a subject. For instance, sports journalist could be broken down further each sport could have a subcategory.