How is cost basis for rental real estate computed on the Depreciation and Amortization report? What is the formula for establishing the ”depreciable basis”?

I sold rental real estate this year that I owned for 6 years and that I have had less than 14 days of personal use each year – don’t understand how my cost basis is being reported.

How is cost basis for rental real estate computed on the Depreciation and Amortization report?  The “depreciable basis” column has been reduced by a different amount than the depreciation each year.  What is the formula for establishing the ”depreciable  basis”?