Airnode serves an API to a blockchain according to Oracle Integration Specifications (OIS). An API is composed of operations, which represent individual functionalities that an API offers. OIS maps each API operation to an endpoint, which can be thought of as an Airnode operation. The endpoints that an Airnode will serve over the request–response protocol are listed under triggers of config.json.
An alternative method to create an endpointId is to use the Admin CLI to derive the endpoint ID.
Note that this means that an endpointId is not unique, and two Airnodes can serve equivalent endpoints using the same ID (in fact, this is the desired outcome). This is not an issue, as requests are made with a airnode (Airnode's address) and endpointId pair.