“Copy as Link” copies a link to the current document or resource. “Current” means the document or resource which is selected or open in the frontmost active window.
Different types of resources produce different types of links. Websites and pages will produce familiar http(s) URLs.
Most other links will be Hook URLs, starting “hook://” and formed “hook://scheme/identifier”. The “scheme” portion of the URL corresponds to the application or type of resource. E.g. links to emails look like “hook://email/identifer” and files are “hook://file/identifier”
“Copy as Link” writes both a plain text and rich text (rtf) link to the clipboard. The rich text link is a hyperlink and the plain text link is just the URL.
macOS will paste the format which matches the text field, so if you paste into the subject of an email in Mail.app it will paste plain text, and if you paste into the body it will paste rich text.