"The statutory debt limit applies to almost all federal debt. The limit applies to federal debt held by the public (that is, debt held outside the federal government itself) and to federal debt held by the government's own accounts. Federal trust funds, such as Social Security, Medicare, Transportation, and Civil Service Retirement accounts, hold most of this internally held debt. The government's surpluses or deficits determine essentially all of the change in debt held by the public. The government's on-budget fiscal balance, which excludes a U.S. Postal Service net surplus or deficit and a large Social Security surplus of payroll taxes net of paid benefits, does not directly affect debt held in government accounts. Increases or decreases in debt held by government accounts result from net financial flows into accounts holding the debt, such as the Social Security Trust Fund. Legal requirements and government accounting practices also affect levels of debt held by government accounts. On August 2, 2011, President Obama signed into law the Budget Control Act of 2011 (BCA; S. 365), after an extended debt limit episode. The federal debt reached its statutory limit on May 16, 2011, prompting Treasury Secretary Timothy Geithner to declare a debt issuance suspension period, allowing certain extraordinary measures to extend Treasury's borrowing capacity. The BCA included provisions aimed at deficit reduction and would allow the debt limit to rise between $2,100 billion and $2,400 billion in three stages, with the latter two subject to congressional disapproval. All three increases, totaling $2,100 billion, have occurred."
CRS Report for Congress, RL31967
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html