Each project is assigned a quota which defines how much resources a project may use.
The Cloud Service accounts on actual usage of resources not on the quotas assigned to them.
The resources used may have provide a different Quality of Service, depending on the characteristics of the resource. Resources like cores, may have different costs depending if there is a physical, dedicated or shared allocation. Volumes and Fileshares use different backends with spinning drives or solid-state drives that have different associated cost.
All this usage information is gathered, then aggregated by chargegroups and finally delivered to the IT Service Costing API.