OK, so I've played around with this a bit and think I've figured it out. It's what I think my Referrer System, which I built in 2002, would have become had it grown up (it peaked at 800K hits per day, and I didn't have the resources to sustain that). The idea here is that, if you read something and you want to comment, you comment on your own page, not the page you're reading. Then what happens is that your system sends the other system a notification saying you've added a comment (you can also send it manually). The other system can then do whatever it wants with that notification (a typical use would be to list your comment along with others under the article). None of the documentation I've seen so far is particularly clear (and as usual there us no Perl reference code). Here's an explanation and code from Ben Werdmuller, here's more from Indieweb, and here's a service that (confusingly) supports it called Bridgely. I hope it's successful, because it creates a distributed web, not one centralized on social networks.