Here's what GPO says about the Core Documents service:
Core Documents of U.S. Democracy
To provide American citizens direct online access to the basic Federal Government documents that define our democratic society, a core group of current and historical Government publications is being made available for free, permanent, public access via GPO Access. These titles contain information which is vital to the democratic process and critical to an informed electorate. They support the public's right to know about the essential activities of their Government. Immediate, online access to authenticated versions of these Core Documents of Democracy increases in importance as Americans grow ever more dependent on remote electronic access to basic information resources -- both past and present.