Sure, this is just label, this ssh-keygen call creates an Resident Key entry (you can see them using e.g. Chromium: chrome://settings/securityKeys "sign-in data"). So this parameter: "-Oapplication=ssh:foobar" is setting the option (-O) for the "application" label, which is in this case "ssh:foobar". So far I know this is a generic mechanism in ssh-keygen and the component that talks to the Nitrokey interprets it. But I also cannot say where to find a consistent list of all these options documented, I also just "know" it, but no idea where it comes from. If you find a source, please share it...