$443 million of federal tax credits potentially available to offset future tax liabilities. Our federal tax credits are primarily related to the U.S. federal research and development credit, which expired in 2014. 

See Item 8 of Part II, “Financial Statements and Supplementary Data-Note 11-Income Taxes” for additional information. 

Non-GAAP Financial Measures 

Regulation G, Conditions for Use of Non-GAAP Financial Measures, and other SEC regulations define and prescribe the conditions for use of certain non-GAAP financial information. Our measures of “Free cash flow,” operating expenses with and without stock-based compensation, and the effect of foreign exchange rates on our consolidated statements of operations, meet the definition of non-GAAP financial measures. 

We provide multiple measures of free cash flow, and ratios based on them, because we believe these measures provide additional perspective on the impact of acquiring property and equipment with cash and through capital and finance leases. 

Free cash flow is cash flow from operations reduced by “Purchases of property and equipment, including internal-use software and website development,” which are included in cash flow from investing activities. 

Free cash flow less finance principal lease repayments and capital acquired under capital leases is free cash flow reduced by “Principal repayments of finance lease obligations,” which are included in cash flow from financing activities, and property and equipment acquired under capital leases. In this measure, property and equipment acquired under capital leases is reflected as if these assets had been acquired with cash. 

All of these free cash flow measures have limitations as they omit certain components of the overall cash flow statement and do not represent the residual cash flow available for discretionary expenditures. For example, these measures of free cash flow do not incorporate the portion of payments representing principal reductions of debt or cash payments for business acquisitions. Additionally, our mix of property and equipment acquisitions with cash or other financing options may change over time. Therefore, we believe it is important to view free cash flow measures only as a complement to our entire consolidated statements of cash flows. 

Operating expenses with and without stock-based compensation is provided to show the impact of stock-based compensation, which is non-cash and excluded from our internal operating plans and measurement of financial performance (although we consider the dilutive impact to our shareholders when awarding stock-based compensation and value such awards accordingly). In addition, unlike other centrally-incurred operating costs, stock-based compensation is not allocated to segment results and therefore excluding it from operating expenses is consistent with our segment presentation in our footnotes to the consolidated financial statements. 

Operating expenses without stock-based compensation has limitations since it does not include all expenses primarily related to our workforce. More specifically, if we did not pay out a portion of our compensation in the form of stock-based