Our measures of Adjusted EBITDA, FFO and AFFO may not be comparable to similarly titled measures of other companies, including other companies in the tower sector or those reported by REITs. FFO and AFFO presented are not necessarily indicative of the operating results that would have been achieved had we converted to a REIT, nor are they necessarily indicative of future financial position or operating results. Our FFO and AFFO may not be comparable to those reported in accordance with National Association of Real Estate Investment Trusts, including as a result of our adjustment to the income tax provision to reflect our estimate of the cash taxes had we been a REIT.
Adjusted EBITDA, FFO and AFFO are presented as additional information because management believes these measures are useful indicators of the financial performance of our core businesses. In addition, Adjusted EBITDA is a measure of current financial performance used in our debt covenant calculations.
Adjusted EBITDA. Crown Castle defines Adjusted EBITDA as net income (loss) plus restructuring charges (credits), asset write-down charges, acquisition and integration costs, depreciation, amortization and accretion, amortization of prepaid lease purchase price adjustments, interest expense and amortization of deferred financing costs, gains (losses) on retirement of long-term obligations, net gain (loss) on interest rate swaps, impairment of available-for-sale securities, interest income, other income (expense), benefit (provision) for income taxes, cumulative effect of change in accounting principle, income (loss) from discontinued operations and stock-based compensation expense.
Funds from Operations. Crown Castle defines Funds from Operations as net income plus adjusted tax provision plus real estate depreciation, amortization and accretion.Adjusted Funds from Operations. Crown Castle defines Adjusted Funds from Operations as Funds from Operations before straight-line revenue, straight-line expense, stock-based compensation expense, non-real estate related depreciation, amortization and accretion, amortization of deferred financing costs, debt discounts, and interest rate swaps, other (income) and expense, gain (loss) on retirement of long-term obligations, net gain (loss) on interest rate swaps, acquisition and integration costs, asset write-down charges and less capital improvement capital expenditures and corporate capital expenditures. Sustaining capital expenditures. Crown Castle defines sustaining capital expenditures as either (1) corporate related capital improvements, such as information technology equipment and office equipment or (2) capital improvements to tower sites that enable our customers' ongoing quiet enjoyment of the tower. The tables set forth below reconcile these non-GAAP financial measures to comparable GAAP financial measures. The components in these tables may not sum to the total due to rounding.