A download's status refers to its availability in the past, present and/or future.
Available
An available download has been officially released by its creator and can be currently downloaded.
An unavailable download has been officially released by its creator, however, it cannot be downloaded currently for some reason. Common reasons include broken links (links not working), and forums offline or deleted.
Unreleased
An unreleased download has been revealed by its creator or there is some other form to know of its existence, however, it has not been released for some reason. It could be unfinished, have bugs the creator cannot fix, will be included in an upcoming pack with other content, or the creator simply has not wanted to release it yet.
Special Conditions
Special conditions need to be met in order to be able to get some downloads. Some are contest prizes given to only one person or a few people, while some need to be "bought" via forum points or similars, among others.