GUID

A  GUID  or Globally Unique Identifier is a (typically) 128-bit number to uniquely reference or identify something; on the IndieWeb, GUIDs are encouraged in some feed file formats as supposedly more persistent than permalinks but in practice GUIDs have been less portable and harder to preserve/migrate/restore than permalinks.

Less portable than permalinks
"So I redesigned my blog. [...] After 14 years of blogging, I switched from MovableType to WordPress. [...] It took some effort, but I preserved the links to everything I’ve ever written—472 posts and 15,891 links. [...] The RSS feeds should redirect appropriately, though inevitably marking everything as new because I couldn’t migrate GUIDs." emphasis added
 * Per http://waxy.org/2016/11/redesigning-waxy-2016-edition/