This page is part of the FHIR Specification (v1.6.0: STU 3 Ballot 4). The current version which supercedes this version is 4.0.1. For a full list of available versions, see the Directory of published versions 

The URL "http://hl7.org/fhir/sid/us-ssn" is the FHIR system identifier for Social Security Numbers Issued by the US government.
For further information, see http://www.ssa.gov/ssnumber/
.