This excerpt taken from the BA 10-K filed Feb 28, 2005.
Provisions for federal, state and foreign income taxes are calculated on reported pre-tax earnings based on current tax law and also include, in the current period, the cumulative effect of any changes in tax rates from those used previously in determining deferred tax assets and liabilities. Such provisions differ from the amounts currently receivable or payable because certain items of income and expense are recognized in different time periods for financial reporting purposes than for income tax purposes. Significant judgment is required in determining income tax provisions and evaluating tax positions. We establish reserves for income tax when, despite the belief that our tax positions are fully supportable, there remain certain positions that are probable to be challenged and possibly disallowed by various authorities. The consolidated tax provision and related accruals include the impact of such reasonably estimable losses and related interest as deemed appropriate. To the extent that the probable tax outcome of these matters changes, such changes in estimate will impact the income tax provision in the period in which such determination is made.