On the Internet Archive data dumps for Stack Exchange, there is a file called PostLinks.xml. I know from viewing the files it contains the following fields: Id, CreationDate, PostId, RelatedPostId, and PostLinkTypeId. PostLinkTypeId can have values of either "1" (signifying Linked) or "3" (signifying Duplicate).
I am not clear as to what the purpose of this file is. It seems it allows one to identify linked or duplicate posts. Why would a person want to identify such things?
I also noticed that closed forums (i.e. closed private betas and closed public betas) do not have the PostLinks.xml file in their data dump. Is there a reason for this?