Atlassian Connect enables developers to build add-ons for the Atlassian products. You can use the API to insert content into the UI, Make calls to the REST API, and receive webhooks.
Authentication is done either with OAuth or via a bearer token. Further details can be found here https://developer.atlassian.com/static/connect/docs/latest/index.html
The terms and conditions can be found here https://developer.atlassian.com/market/atlassian-developer-terms
The status page for the API can be found here https://developer.atlassian.com/market/atlassian-connect-status
Authentication with the Atlassian Connect service is done via JWT (JSON Web Token).