Gross federal debt consists of debt held by the public
and debt issued to government accounts (for example,
the Social Security trust funds). The latter type of debt
does not directly affect the economy and has no net
effect on the budget. In CBO’s projections, debt held
by the public increases by $9.8 trillion between the
end of 2016 and the end of 2026, and debt held by
government accounts rises by $0.1 trillion. As a result,
gross federal debt is projected to rise by $10.0 trillion
over that period and to total $29.3 trillion at the end
of 2026. About one-fifth of that sum would be debt
held by government accounts.
https://www.cbo.gov/sites/default/files/51118-2016-01-BudgetProjections3.xlsx