API calls refer to the number of requests that reach our servers. It includes requests when people visit a project Timeline, load Widget data but also requests sent to our GraphQL API and events received from or sent to Zapier integrations.
However, requests made while using the Noticeable website (https://noticeable.io) do not count against the quota. Also, as you mentioned, requests are cached by default by a first-class Content Delivery Network (CDN). All CDN requests are free of charge and do not count against the quota.
In other words, not only direct API calls counts but also indirect ones.
Your comment lets me think "API calls" is misleading. Do you think "requests" would help to clarify? or an on hover popup with the explanations above?