The responsibility to prove entries, deductions, and statements made on your tax returns is known as the burden of proof. One must be able to prove certain elements of expenses in order to deduct them. The IRS Restructuring and Reform Act of 1998 shifted the burden of proof to the IRS in civil tax matters. This act, however, does not relieve the taxpayer of the need to keep proper records.

Review the siteThe Internal Revenue Service Restructuring and Reform Act of 1998, which describes the restructuring of the IRS to a customer service organization. Then, answer this question: Why would the IRS bear the burden of proof yet require the taxpayer to keep careful records?

https://www.treasury.gov/tigta/iereports/2010reports/2010IER002fr.html