Postman Collections provide another type of technical contract for your API, but one that is designed for execution, testing, and automation. The Postman Collection format is a format developed by Postman as part of their API client and testing solution, but because it can be important and exported, and run in CI/CD pipelines, it has become a ubiquitous representation of what an API can do.
Similar to an OpenAPI, and often times generated from an OpenAPI, a Postman collections is only available in a JSON format and is tends to be less human readable than an OpenAPI YAML technical contract, and is only used in services and tools–rarely hand edited. A Postman Collection provides many of the same technical details as an OpenAPI, but does not have the JSON Schema for requests and responses.
Postman Collections are an excellent companion for an OpenAPI, providing many modular derivatives of an API in executable unites that represent specific business workflows, tests, or other types of automation using APIs. Postman Collections are a great way to automate the technical details of an API contract, brining more of the data exhaust from across the API lifecycle into the API contract, tracking on API testing results, gateway configuration, and other key business or technical details of doing business with an API.