section 179 is limited to business income before the deduction for 179 depreciation. most items that qualify for section 179 depreciation qualify for bonus depreciation (section 168(k)). bonus depreciation is not limited to business income. in figuring the limit on section 179 depreciation section 168(k) depreciation, also referred to as special depreciation, is subtracted first from business income.
for clarification of these 179 and 168(k) rules see IRS PUB 946
https://www.irs.gov/pub/irs-pdf/p946.pdf
