Hey Eson,
If I read the documentation correctly, the API seems to be for the Mendix users (the username password you use to login to your Mendix developer portal) and Mx platform. So if you want to expose the data or update this data related to Mendix users then the User Management API should be used I guess.
If you want to be able to perform CRUD operations for your specific entity in Mendix, I think you need to create your own APIs.
Take a look at the following if you need reference:
https://docs.mendix.com/howto/integration/publish-rest-service
Hope this helps!