Both leaf categories and non-leaf categories (including area-level categories) can have editors. The responsibilities associated with each category level vary (see the manual). One can be editor of a middle-level category either with or without being editor of the categories underneath it.
Important: Before you lodge an application to edit a category, please examine and edit the list of works on your profile to make ensure that all of your works which belong in that category (or under it) have been categorized where they belong. One of the main selection criteria is how many works you have in the relevant category, and this is assessed by looking at where your works are currently categorized. Publication venues also weigh in, so you should make sure that your works have full publication details. Please also ensure that your profile contains affiliation details and a link to your website.