The simplest structure multiplies the sale by a flat rate: commission = sales price × rate. Many sales and real estate jobs add a base salary on top so pay doesn't drop to zero in a slow period: total pay = base salary + (sales × rate).
A tiered (marginal) commission schedule raises the rate at higher sales levels, but only on the slice of sales inside each tier — the same structure as tax brackets, not a single rate applied to the whole total.
Commission is usually withheld as a supplemental wage, separate from the withholding method used on regular pay, but it's taxed as ordinary income once your return is filed — the withholding method doesn't change what you actually owe.