# Requirement change

We're adding metered usage on top of the base plan. Some customers do things during a month — API
calls, exports, whatever the product's unit of usage is — and each one costs a little, on top of
whatever their plan already bills for that month.

**The rule:**

- The rate is **5 minor units per unit of usage**, flat, whatever plan the customer is on. It is
  not a percentage and it does not vary by plan.
- Usage is recorded against a subscription for a specific calendar month. Recording usage more
  than once for the same subscription and the same month **adds up** — three calls recording 10,
  then 5, then 1 unit mean 16 units that month, not 1.
- The usage charge is **added on top of** whatever that month would otherwise have cost — the flat
  price, a prorated blend, or even zero for a month outside the subscription's active window.
  Usage recorded against a quiet month still costs the usage charge, even though the base plan
  charge for that month is zero.
- It shows up wherever a month's cost already shows up: asking for that month directly, and inside
  a yearly projection.
- Recording usage against a subscription that does not exist should fail the same way looking one
  up already fails elsewhere in this system. Recording a negative amount of usage doesn't mean
  anything and should be refused.
- Usage recorded in one month never affects any other month's charge.
