A reference is the information on how to reach an object. A reference contains a list of addresses. An address identifies a communication endpoint (an object). For example, the address of a machine such as nismaster.doc.com, or a user's email address such as firstname.lastname@example.org.
A reference might contain multiple addresses that identify multiple communication endpoints for a single conceptual object or service. For example, a list of addresses might be required because the object is distributed or because the object can be accessed through more than one communication mechanism.
XFN cannot guarantee specific properties of addresses such as their stability, validity, or reachability. A client might be able to look up a name but not be able to use the returned reference because the client might not have support for any of the necessary communication mechanisms or might lack the necessary network connectivity to reach the address. Further, the address might be invalid from that origin or stale; these issues are the province of the convention between the name's binder, the clients, and the service provider specified in the address.