Consumption of a billable service or resource (such as database storage space or bundles of emails sent) provides the basis for some charge models - simple usage, tiered pricing, or volume pricing. To make this work, usage must be tracked in the system and usage charges must be calculated and invoiced. Usage is always billed in arrears - for example, you might bill customers in February for their January usage. Usage can be billed on a recurring monthly, quarterly, semi-annual, or annual basis.
For more information about working with usage data, see Usage.