Credits are how usage is measured. They are tied to the volume of data you fetch.
APIs
The number of credits deducted for each request varies depending on the platform. Read individual API docs for more info.
A minimum of 1 credit is deducted for every successful request, even when no results are present in the response.
Credits are deducted only for successful requests that have a 200 response code.
Scrapers
When using scrapers 1 credit is deducted for each record present in the response.
That means, if your job for the Amazon Reviews Scraper collects 492 reviews, the number of credits used for that job will be 502 i.e. 492 + 10 which is a base fee.