Otto exposes all of the API endpoints it uses to perform it's tasks to developers. The intent is to let developers interact with the server in any way that they see fit.
FileMaker version 16 introduced JSON and full cURL support into the product. That means that the entire Otto API is exposed to FileMaker Scripts, you are free to use your FileMaker skills to create any kind of migration or server automation tool you want to. It puts the power in your hands.
Of course, you can use any other HTTP Client as well to interact with Otto as well. You can build web apps, iOS apps, anything you like.
The API is exposed via a RESTFul JSON API. Authentication is either with Basic Auth or AccessToken.