What is the main purpose of an API specification?

The main purpose of an API specification is to outline the structural design of an API. This includes defining the endpoints, request and response formats, data types, and any rules or constraints governing how the API behaves. By having a clear API specification, developers can understand how to interact with the API effectively, ensuring that integrations are built correctly and that systems can communicate seamlessly. It serves as a contract between the API provider and the consumer, providing detailed documentation that helps facilitate development, testing, and maintenance of the API.

The other options, while related to the environment in which APIs operate, do not capture the fundamental purpose of an API specification. Restricting access to backend systems pertains more to security and authentication controls rather than design. Regulating transaction load across databases involves performance management, which is not directly addressed by an API specification. Creating user interfaces for applications relates primarily to front-end development rather than the structural design of APIs. Thus, focusing on the structural design aligns perfectly with the core function of an API specification.