Along with the giving you the ability to control the over $2 trillion in yearly spending, this simulation gives you the opportunity to adjust what are called officially "tax expenditures" and are unofficially referred to as tax loopholes.
These tax entitlements loom large in the overall budget picture; in fact, the total of all tax expenditures comes to about $980 billion, enough to eliminate the deficit and cut income tax rates across the board by 50% to boot. Tax expenditures cost two and a half times as much as all means-tested direct spending programs.
Since such tax expenditures for specific political goals are paid for by either deficits or higher taxes on the general population, they are little different from general spending, aside from the fact that the money is spent with less accountability and political review.
Such expenditures are tabulated each year by both the Congressional Joint Commitee on Taxation and the Treasury Department along with their annual estimated cost. What distinguishes a "tax expenditure" from a general tax deduction is defined by the Joint Tax Commitee on Taxation as follows:
"Special tax provisions are referred to as tax expenditures because they are considered to be analogous to direct outlay programs...Tax expenditures are most similar to those direct spending programs which have no spending limits, and which available as entitlements."
The tax expenditures in this simulation follow the official tax expenditure lists established by the Office of Management and Budget. So cutting back some of these tax expenditures may slightly overestimate the actual savings the budget would gain in those areas.
Numbers for tax expenditures come from the White House's Analytic Perspectives document, Section 6, on Tax Expenditures at here (PDF)