rel-syndication is a way to discoverably link from your original blog post permalinks to syndicated copies of it on other sites
An original post should hyperlink to syndicated copies (e.g. per POSSE) with a rel value of syndication.
Additionally the microformats2 class name u-syndication should also be placed on such hyperlinks from original posts (inside their h-entry markup/object) to their syndicated copies.
For now, only Social plugin is supported (http://wordpress.org/plugins/social/).