Traditionally, access to the Tessitura API endpoints is controlled at the network layer through IP whitelisting.
This is changing for the new RAMP environment:
https://community.tessituranetwork.com/tessitura_software_forums/f/agile_software_development-forum/27625/external-access-to-the-api-help-documents-and-the-rest-client-in-the-new-cloud-hosting-environment