Copy As Universal Link is a Hookmark Pro feature introduced in Hookmark 5.
It copies the selection as a universal link.
Hookmark’s universal links are particularly useful for (a) sharing links to emails and (b) using in websites like Notion and Google docs. (Notion generates its own universal links. But sometimes you want to include links to other content in Notion itself. Custom links (like OmniFocus:/// and hook:///) that are pasted in Notion and Google Docs are not clickable (they seem to want you to stay in their walled gardens); but those web apps do accept universal links).
Find out more here about Universal Links– what they are, why they are useful, what their limitations are