ZVON > References > XLINK Reference
Go to standard 
attribute: href
for elements: <simple>, <locator>
Standard excerpt:
Locator attribute.
Values: Standard excerpt:
The value of the href attribute must be a URI reference. If the href attribute has a value but that value is empty ("") or has an empty URI portion, it is to be interpreted as identifying the resource in which the href attribute itself appears.

Examples (xlink:xml):