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Rate limits and units
One unit per call at any block age, limits published per plan, and overage that keeps serving rather than stopping.
Units
One request unit per JSON-RPC call, whatever the method and whatever the block age. A batch counts per inner call. WebSocket connections, webhook deliveries and keys are free.
Archive calls cost the same as calls against the tip. Arc produces a block every 0.5s, so the usual convention of treating anything more than 127 blocks behind the tip as archive covers about one minute of history here. Under a multiplier model, reading yesterday bills at the archive rate.
Limits by plan
At the ceiling
A plan that reaches its ceiling keeps serving and bills the overage, because a settlement pipeline that stops on the last day of the month is worse than an invoice larger than planned. Sustained traffic over the plan rate limit throttles with a 429 and a retry-after rather than failing outright.
Accounting headers
x-arcrpc-units reports what a call cost, x-arcrpc-region which region served it, and x-arcrpc-node-build which execution build answered. Log the first if you bill your own customers by usage.