The denomination to round to. For payroll and budgeting you might use $1; for estimates you might use $10 or $100.
Standard half-up is the most common rule: 0.50 rounds up, below 0.50 rounds down. Ceiling always rounds up (conservative budgeting). Floor always rounds down (conservative revenue estimates). Banker's rounding rounds 0.50 to the nearest even number, reducing statistical bias in large data sets.