How is XRI resolution and XRDS documents extensible?
XRDS documents are designed to be pure container elements for XRDs as well as elements from any other XML namespace can be added. XRDs, for their part, are a very simple and highly extensible resource descriptor format. The XRD schema has a very small set of defined elements used for XRI resolution and service endpoint description. However, there are numerous places where arbitrary XML may be added. Examples of such XML are service attributes, alternate identifiers, RDF statements, security tokens, or almost any other metadata about the described resource. For full details see XRI Resolution 2.0, Section 17.