A category is a collection of articles. Diary++X shows categories at the category list.
Diary category is a collection of dated articles. It contains year categories, year category contains month categories.
Memo category is a collection of undated articles. It contains chapter categories.
Bookmark category is a collection of bookmarks. It contains bookmark folders, bookmarks, smart bookmarks as sub categories.
History category is a collection of daily navigation articles. It contains daily history items.