Resource billing plans charge you based on the amount of resources (currently CPU and RAM) that your site uses. There are two types of resource-based billing: individual and stochastic. For both types, your site will be assessed a charge based on the resource accounting units it accrues.
Most newer sites types (mainly PHP 5.4 and later) support individual resource billing. For these, our system keeps track of the amount of CPU and RAM used by that individual site and generates accounting records based on that usage.
If you have an older site that is not using a resource-billing server type (aka "legacy billing"), its resource usage may still be measured for our internal use in load balancing, so you may see resource usage information in the control panel, but you will not be charged for it. Your storage costs, however, will be considerably higher.