System URI: https://bluebutton.cms.gov/resources/identifier/hicn-hash
Prior to the introduction of the Medicare beneficiary Identifier (MBI), the Health Insurance Claim Number (HICN) acted as the Medicare beneficiary’s identification number and was used for paying claims and for determining eligibility for services across multiple entities (e.g. Social Security Administration (SSA), Railroad Retirement Board (RRB), States, Medicare providers and health plans, etc.)
With a few exceptions, the HICN is no longer used. See https://www.cms.gov/medicare/new-medicare-card for more details.