A trader runs a business of profiting from short-term price movements rather than from dividends or long-run appreciation, trading substantially and continuously, judged by holding periods, frequency and size, livelihood dependence, and time devoted. Missing those tests leaves the taxpayer an investor, whatever the brokerage statements are called. Without the mark-to-market election, even a trader's gains stay capital on Form 8949 and Schedule D under the loss limits; the business shows only in the expenses, which move to Schedule C, and in the exemption of the gains from self-employment tax. With the section 475(f) mark-to-market election, all trading gains and losses are ordinary in Part II of Form 4797 instead. A wash-sale loss stays nondeductible unless it was incurred in the ordinary course of business as a dealer in stock or securities.
Worked example
A former engineer trades most mornings, holds positions for hours, places 700 trades a year, and supports herself on the profits: a trader. Her $30,000 of gains go on Form 8949 and Schedule D, her $2,400 of data and platform fees go on Schedule C, and no self-employment tax touches the gains. Her brother, who buys index funds quarterly and holds for years, is an investor on the same markets.