Help: Post Relationships
Every post can have a parent. Any post that has a parent will not show up in the main listing. However, the post will appear again if a user does any kind of tag search. This makes it useful for things like doujinshi or manga pages, since now you can upload entire chapters without cluttering up the listing.
To use this field, simply enter the id number of the parent post when you upload or edit a post. To search for all the children of a parent post, you can do a tag search for post:nnnn, where nnnn is the id number of the parent post.