The IRS has rules that describe proper record-keeping for tax deducted expenses.  Substantiating is an IRS term that refers to proper record-keeping.  When you use EZPerDiem you can easily substantiate expenses by clicking the Expense Icon on the Layover Entry Page next to the date the expense was incurred.

Substantiating is optional in EZPerDiem, but if your return is audited by the IRS, expenses that you deduct without substantiating might be removed and subject to tax and penalty by the IRS.  Using EZPerDiem to substantiate your flight crew tax deductions is free of charge.

EZPerDiem strongly recommends that you properly substantiate your expenses.