One issue that's cropped up in Salmon is how to identify authors. Mostly, feed syndication protocols hand-wave about this and at most specify a field for an email address. Atom goes one step further and provides a way to specify a URI for an author, which could be a web site or profile page.
The main things that Salmon needs are (1) a stable, user-controlled identifier that can be used to correlate messages and (2) a way to jump from that identifier to a public key used for signing salmon. The existing atom:author/uri element works fine for #1. Webfinger+LRDD discovery can then take over for #2. What comes out of that process is a public key suitable for verifying the provenance (authorship) of a salmon.
Now for the details. It turns out that the flow for Webfinger is pretty stable, so any time you have an email like identifier for an author you can just slap "acct:" in front and things will Just Work. But I don't want to limit authorship only to acct: URIs - I should be able to use any reasonable URI in the author/uri field and have things work. By this I mean any URI that I can perform the right flavor of XRD discovery on and get out the public key. It turns out that the existing specs cover 90% of this flow, but some things, like the order in which to look for links, remain unspecified. If they remain unspecified by the time Salmon gets real deployment, then Salmon will need to fill in the gaps in its specification. I'm hoping that's not necessary but it won't be a blocker if it is.
Also, there isn't an existing agreed-upon term that covers both Webfinger identifiers and the kind of URIs that you can perform LRDD/XRD discovery on, such as https://myprofile.com/myname. Personally I would prefer to call them all Webfinger IDs but for now I'm calling them all discoverable URIs. Which is a terrible name since, obviously, you don't need to discover the URI as that's what you're starting from. Hopefully that will be motivation to get people to agree on a better name. Also, this is obviously useful for more than just user account identifiers. To start with, automatic services and sites will want to be able to participate as well. For example, did you know that the Blogger service has a public signing key? It would be good to have that published somewhere more discoverable than this blog post.