The place to start with defining any API is properly defining who the target audience is. Ideally the who is a person or group of people, defined as real-world teams or customers, or more generalized demographics. The goal is to project an actual image into the heads of teams producing APIs, using the definition of who as a north star.
Each API may have a single target audience, or there may be many who are putting an API to work. Try to keep an eye on the who portion of the use case as new consumers come on board, and especially as you deliver new features. Who is consuming an API may evolve over time, shifting from the original target audience established when it was first launched.
Developing an ongoing awareness of who is using an API and aligning it with a formal definition of use cases included in an API’s contract is how you ground the design of an API, and keep on track with each release. Defining who is using an API sets the stage for eventually also identifying more trusted beta users who can be included in early releases, helping better align use cases with actual API releases.