The purpose can be documented using invoices and receipts. In most cases, a cancelled check or credit card statement isn’t enough to substantiate the purpose. In some cases, there isn’t enough information on the receipt to document a deductible purpose. In that case, it is a good idea to keep a copy of promotional material explaining the expenditure. (I am helping with an audit where an agent is proposing to disallow deductions for a business coach despite a letter of explanation from the coach and having invoices to document the expenses!)