Usage allowance that is automatically granted to the customer at the beginning of the billing period and automatically expires at the end of the period. May be limited to specific charges.

Example :

The customer pays $1,000 upfront every month, which allows them to use 500 billing units for any metric included in their plan (e.g. may use 250 units of two metrics, or 100 units of metric A and 400 units of metric B). At the end of the billing period, the unused allowance is lost.

Similar use case with credits: https://getlago.canny.io/feature-requests/p/periodic-allowance-credits