Today the CRM pricing engine supports 2 forms of volume discounting, both based on the cumulative quantity:
1) A fixed amount based on the cumulative quantity: 1-5=$0 discount, 6-10-$5.00 discount, 11-20=$10.00 discount
2) A percentage based on the cumulative quantity: 1-5=0% discount , 6-10=15% discount, 11-20=20% discount

Both methods create pricing gaps at the edge of the each level where increasing the quantity by 1, provides significant rebate.

In "tranche" discounting, the rebates only apply to the quantities within the level: The first 5 get no discount, the next 5 are discounted by 15% (or $5.00), the next 10 get discounted by 20% (or $10.00).