Tax law
As a result, the computation of the tax expense is considerably more complex. Tax law may provide for different treatment (from GAAP) of items of income and expenses as a result of tax policy. The differences may be of permanent or temporary nature. Permanent items are in the form of non taxable income and non taxable expenses. Things such as expenses considered not deductible by taxing authorities ("add backs"), the range of tax rates applicable to various levels of income, different tax rates in different jurisdictions, multiple layers of tax on income, and other issues.