This excerpt taken from the SIRI 10-K filed Mar 1, 2007.
Results of Operation
Our discussion of our results of operations, along with the selected financial information in the tables that follow, includes the following non-GAAP financial measures: average monthly churn; SAC, as adjusted, per gross subscriber addition; customer service and billing expenses, as adjusted, per average subscriber; average monthly revenue per subscriber, or ARPU; free cash flow; and adjusted loss from operations. We believe these non-GAAP financial measures provide meaningful supplemental information regarding our operating performance and are used for internal management purposes, when publicly providing the business outlook, and as a means to evaluate period-to-period comparisons. Refer to the footnotes following our discussion of results of operations for the definitions and usefulness of such non-GAAP financial measures.