I think it does both, like it can be a card name or a URL!
Yep!
To be precise, clicking a link in a field sends a "link[]" event to the field with the text of the link contents. If you write some custom scripting, you can make clicking links do whatever you want, much like a button. If you don't write any scripts, Decker's default behavior for the "link[]" event is to attempt to open anything which looks like a complete URL (starting with "http://" or "https://") in a new tab, and anything else is interpreted as a card name to "go[]" to. Additional details can be found in the reference manual under Events.