The <url> tag always requires an @href attribute. Usually this will be a complete address for some external web page, or other external resource. The @@visual attribute is sometimes mandatory, but sometimes optional. It should provide a simplified version of the URL for use in print, or similar situations. Finally, you can provide content for the <url> element, which will become the clickable text in most realizations.
If the <url> element is empty (no content), then the value of the @href attribute or the optional @visual attribute will be the link text, with a preference for the latter. When you instead provide content, you can use PreTeXt elements much like any other piece of text that would occur in a paragraph. In this case, a @visual attribute is now highly recommended, as an alternative to the content, providing information about the actual URL for non-electronic formats like print. A default version of the value of the @href attribute will be used in its absence. This visual version of the URL will appear in a footnote.