It has nothing to do with Google. You can define a link relationship from your sites as anything. i.e. rel="friend", which is completely for humans and not spiders. See: http://gmpg.org/xfn/intro
Let me Google that for you... http://www.w3.org/2008/WebVideo/Annotation/wiki/MultilevelDescriptionReview dc:source, "A related resource from which the described resource is derived." Dublin Core Metadata Initiative lives at http://dublincore.org/ (The "Dublin" part refers to the location of the group first meeting in Dublin, Ohio in 1995)
...I checked the inbound link from Digg to my signal integrity site using HubSpot and it shows that the new Digg dc:source relationship (unlike the old rel="nofollow") does get SEO credit.