Billing type

The cleanest way to see the split is the two purest examples in the catalog, both self-evident from their own names. Per-second billing charges more the longer the clip runs, the way a taxi meter does. Flat billing charges the same number of credits for the job regardless of length, the way a fixed quote does. Between those two poles sits a third shape entirely: billing keyed to the size of the input or output rather than to time at all — a rate per megapixel of image, or per thousand characters of script read aloud.

The confusion this causes is specific and avoidable: people carry an assumption from the last model they used onto the next one without checking. Someone used to a per-second model assumes a longer duration option always costs proportionally more, and is surprised when a flat-billed model charges the same either way. Someone used to a flat-billed image model assumes size does not matter, and is surprised when a per-megapixel model charges more for a larger canvas.

None of this is a setting you choose on a given model — the billing type is fixed per model, decided by how the provider underneath actually charges, not by anything you select in the interface. Checking which basis a model uses before assuming how a longer duration or a bigger canvas will move the credit cost is the only reliable habit, because the catalog genuinely does not use just one.