This code specifies the Social Security Administration (SSA) code for the county of identified through the beneficiary mailing address of the beneficiary.
Each state has a series of codes beginning with '000' for each county within that state. Certain cities within that state have their own code. County codes must be combined with state codes in order to locate the specific county. The coding system is the SSA system, not the Federal Information Processing Standard (FIPS). In some cases, the code may not be the actual county where the beneficiary resides. CMS obtains the mailing address used for cash benefits or the mailing address used for other purposes (for example, premium billing) from Social Security Administration (SSA) and Railroad Retirement Board (RRB) Beneficiary Record Systems.
Source: CMS Common Medicare Environment (CME)