The category tag is four different areas. The first one is deleted/moved. This is when at one point you had a link to a specific fic, but it is no longer available and you are wondering if someone else has an additional link or new link.
The second category is promotion. This is only used when someone is posting an announcement of a new community, or similar announcements. Usually people who use this tag ask the mods if it is okay to post first.
The third and fourth tag are going to be your most commonly used tags and they are the rec tag and specific tag. The rec tag is if you are requesting a different types of fic. For example, "I am looking for fics where New Directions go on vacation." The specific tag is if you are looking for a specific fic that already exists. Example, "I am looking for a fic where the New Directions went to Disneyland and performed."
It would be really awesome if everyone would try to remember to use these tags.