"Total federal debt can increase in two ways. First, debt increases when the government sells debt to the public to finance budget deficits and acquire the financial resources needed to meet its obligations. This increases debt held by the public. Second, debt increases when the federal government issues debt to certain government accounts, such as the Social Security, Medicare, and Transportation trust funds, in exchange for their reported surpluses. This increases debt held by government accounts. The sum of debt held by the public and debt held by government accounts is the total federal debt. Surpluses reduce debt held by the public, while deficits raise it. […] The debt limit rose a third time in less than a year to $12,104 billion with the passage of the American Recovery and Reinvestment Act of 2009 on February 13, 2009 (ARRA; H.R. 1), which was signed into law on February 17, 2009 (P.L. 111-5). Following this measure, the debt limit was subsequently increased by $290 billion to $12,394 billion (P.L. 111-123) in a stand-alone debt limit bill on December 28, 2009, and by $1.9 trillion to $14,294 billion on February 12, 2010 (P.L. 111-139) as part of a package that also contained the Statutory Pay-As-You-Go Act of 2010. Following enactment of the Budget Control Act of 2011 (P.L. 112-25) and the submission of a certification letter from President Obama to the Congress, the debt limit was increased on August 2, 2011, by $400 billion, to $14,694 billion."
CRS Report for Congress, RL31967