The Link User Item adds a link to the current chapter as a list item.
The Title property defines the text of the link. It also identifies the User Item in the User Items list.
See Enabled in the Common Properties section of the User Items help page.
The URL property identifies the destination for the link.
The URL should be an absolute URL, a full link including the HTTP or HTTPS protocol, the domain name, etc.
Most modern e-readers will open the URL using the device's default browser or using a built-in browser. Some older e-readers may not be able to open the URL or will use an inferior, built-in browser, but those e-readers are disappearing, so don't be too concerned about it.
The URL may also be a relative URL referring to an XHTML page in the book, but this is an advanced usage and is not recommended for most users.
The Description text will appear next to the link when the link appears in lists in a Content Chapter.