Residential property is property rented out for nontransient living purposes.
That includes offices, stores, warehouses, hotels and motels, etc. It also includes the portion of a home used for business. That's to avoid waiting until the end of the year to buy the property, yet secure a deduction for the whole year. When calculating the total basis of property placed in service, exclude any on which you took the Section 179 expense election. For real estate, a mid-month convention is used.