Now I'm trying to figure out how a waitress would have Line 21 income.

A waitress is almost positively an employee.  All taxable pay should be on the W-2.  If it is not on the W-2, the W-2 should be corrected or Form 4852 should be filed.

If employer was treating her as an Independent Contractor, Form 8919 should be filled out.

If for some weird circumstance she really is an Independent Contractor, it would be a business on Schedule C.