With the Graduated tier type, the customer pays the unit price at each tier before moving up to the next tier/price. United States income tax works this way at the federal level (and is quite a non-optional subscription!). In the example below, a customer will pay nothing for their first 100 texts, will be charged €0.05 for each of the next 400 texts, and when they hit 500, they’ll be charged €0.10 for each text thereafter.
With the Volume tier type, the customer’s unit price for their entire consumption depends upon the usage tier they fall into. Some utility companies operate this way to incentivize lower usage. In the example below, a customer whose electric consumption falls between 0-2000 kWh will pay €0.13 per kWh. A customer whose usage falls between 2000-4000 kWh will pay €0.15 per kWh, and a customer whose usage exceeds 4000 kWh pays €0.17 per kWh.