This excerpt taken from the PBR 6-K filed Mar 4, 2008.
3.1 Determination of net income, current and non-current assets and liabilities
Net income is determined on the accrual basis and includes income, charges and monetary and exchange variations, at official indexes or rates, calculated on current and non-current assets and liabilities, including, when applicable, the effects of adjustments of assets to market or net realizable values, as well as the provision for doubtful debts, in an amount considered sufficient to cover possible losses on accounts receivable.
Sales revenue is recognized in the income statement when all the risks and benefits inherent to the product are transferred to the buyer. Revenue from services is recognized in the income statement for the period in which it is received.