Substantiated expenses are expenses that have documentary evidence to support them.  It is very important for flight crewmembers to properly document aviation expenses that they deduct from their taxes so that they can be validated if a tax return is audited by the IRS.  See tax audit.  Aviation-related expenses that are deducted from a flight crewmember's tax return must be substantiated for the airline tax deduction to be valid.  A substantiated expense is one that a flight crewmember can prove through the use of adequate records.  IRS Publication 463 describes the level of recordkeeping required by flight crewmembers.