Below the line deductions are tax deductions or adjustments that modify a taxpayer's adjusted gross income and are used to calculate a taxpayer's taxable income.  The so-called "line" is AGI (Adjusted Gross Income).  Therefore, tax deductions that occur after AGI is calculated are called below-the-line deductions.  Any deductions that modify gross income and occur before AGI is calculated are called above-the-line deductions.  Below the line deductions can be one of two forms (either itemized deductions or the standard deduction).

Below the line deductions are used in the following basic calculation:

Adjustable Gross Income - Below the Line Deductions = Tax Income

Employee business expenses make up only one part of airline pilots' or fight attendants' itemized tax deductions, and itemized deductions are below-the-line deductions.  All of the itemized deductions that pilots and flight attendants write-off get entered on IRS Form 2106, so you can think of IRS Form 2106 essentially as one big itemized deduction.  That is the reason that only pilots and flight attendants who itemize can write their airline job expenses.