There are two standards for linking and referencing in the XML world: XLink and XPointer:

• XLink, the XML Linking Language, defines a variety of ways to link different resources together. You can do normal point-to-point links (as with the HTML <a> element) or extended links, which can include multipoint links, links through third parties, and rules that define what it means to follow a given link. The XLink standard is at w3.org/TR/xlink/.
• XPointer, the XML Pointer Language, uses XPath as a way to reference other resources. It also includes some extensions to XPath. You can find the spec at www.w3.org/TR/xptr/.