Worked examples
One rate: sales of 50,000 at 5% earn 2,500 commission.
Two tiers: 3% up to a 30,000 target and 6% above it. On 50,000 in sales, the first 30,000 earns 900 and the remaining 20,000 earns 1,200, so commission is 2,100.
In a tiered plan, only the sales above the target get the higher rate. This is the most common way tiered commission works, but check how your own plan is written.