OGC Best Practices for URNs

a best practice document that describes a standard mechanism for using URNs to reference various abstract concepts and resources, especially those relating to coordinate reference systems (e.g. datums, ellipsoids, prime meridians, etc). The URNs follow the format "urn:ogc:def:crs:EPSG:6.3:26986" (for a CRS defined in the EPSG codespace, in this example)