(NewsNation) — As tax season approaches, millions of Americans qualify to file their taxes for free — but only if they have what’s known as a “simple tax return.”

A simple tax return typically includes only basic income and deduction elements, according to tax preparation experts. The key components usually include:

• W-2 wage income from an employer
• Limited interest and dividend income
• Standard deductions
• Basic tax credits, such as the child tax credit
• Unemployment income
• Earned income tax credit

However, once taxpayers enter more complicated financial territory, they generally need to file a complex return and may have to pay for tax preparation services. Situations requiring a complex return include:

• Self-employment or freelance income (1099 forms)
• Small business ownership
• Rental property income
• Investment earnings from stocks and bonds
• Itemized deductions, such as mortgage interest
• Multiple sources of income

Tax preparation companies define simple returns differently, but most free filing services are limited to basic Form 1040 returns. For example, TurboTax’s Free Edition is available to approximately 37% of taxpayers who file simple returns.

For those who qualify, several free filing options exist.

The IRS offers Direct File, a no-cost filing system available in more than 20 states. Additionally, the Volunteer Income Tax Assistance program provides free tax preparation for people who generally make $67,000 or less, have disabilities or have limited English proficiency.

The tax filing deadline for most Americans is April 15.