The category is a broad label assigned to each agenda item. By default, the
category is simply derived from the file name. You can specify a file-wide
category using the in-buffer setting #+CATEGORY: Value outside of a heading. A
different in-buffer category can be added to a heading, or you can give each entry
its own category with the :CATEGORY:Value property. The AGENDA VIEW uses the
category as the first item in a line.