We are offering different types of Application Programming Interfaces. Depending on the use cases you need to cover, you are free to choose which you prefer.
For Authentication we are using OAuth2 by default. If there is nothing specific mentioned, this is our way of authenticating.
The Backend SDK can be used in direct server-to-server communication. It offers an abstraction layer onto of our datamodel.
The Mobile SDK is used by our SDKs for the mobile clients.
The Basic SDK is only for specific purposes and should be considered instable.