The publication section

The publication section allows for API documentation to be created and then published in the developer portal. The documentation consists of two main parts:

1.  API overview documentation: Simply put, free text to help describe the functionality and usage of the API. It can be HTML or Markdown text.
2. API specification documentation: The API Blueprint or Swagger specification document for the API created in Apiary:

API implementation page–publication section

APIs can be specified in a number of ways, as opposed to web services where the main mechanism to define a service is by creating a WSDL (web service description language) document. For APIs, the most popular specifications are Swagger and API Blueprint. Apiary supports both.

Recommend reading the following article for further information on the topic:
https://nordicapis.com/top-specification-formats-for-rest-apis