It opens a link to a post in its own instance. So for example
this is how your comments looks for me with this button. As you can see I’m not logged in, because it’s a different instance compared to what I use. This is what things look like when I click the chain icon to the left of it:
Here I’m logged in, because it’s your comment opened through my instance.
I blame all three + the driver again for buying this stupid fucking truck they probably don’t even need and won’t benefit them 99% time. But hey, it excels at killing children in driveways, so that’s something.