Plans, subscriptions and recurring documents that raise themselves on their own schedule, plus metering for what a customer actually used and prepaid balances for the ones who pay up front.
Two engines, one ledger. Recurring documents repeat a fixed shape; subscriptions carry plans, usage and prepaid tokens. Both post like any other invoice.
For everything billed on a rhythm — rent, retainers, an annual maintenance contract.
For the model where a customer buys credit and spends it over time.
The schedule raises the document, the meter prices it, and the ledger has it before anyone opens a spreadsheet.