Changes

It is possible that the parent document is the parent application - that is, the first patent application filed in regards to an invention. That would explain why Reissue also has several fields related to a child document - a child document could be a child application, which is filed while a parent application is still pending. So in this case, a child document of a reissue patent would be an application regarding the same patent that was filed while the reissue application was still pending. A child document is either a continuation, disclosure, or continuation-in-part application ([http://www.patenttrademarkblog.com/parent-and-child-patent-applications/ source]).