Final answer:
The documentation requirements for mixed motive expenses involve detailed records, distinguishing between business and personal use, which include date, amount, business purpose, and supporting evidence such as receipts or logs.
Step-by-step explanation:
The documentation requirements to substantiate the business portion of mixed motive expenses include keeping detailed records that can clearly distinguish between the business and personal components of the expense. This documentation should have the date, amount, and business purpose of the expenditure along with evidence that supports the business use, such as receipts, logs, or written explanations. For transportation expenses, for instance, mileage logs noting the purpose of each trip, the miles driven, and the total amount of time used for business as opposed to personal activities are essential to provide accurate substantiation.