I am not sure if any issues you are facing. RingCentral allows to create their application and when an application is created, we can consume various RingCentral APIs as per our need and as per the application is configured.
There is a flexibility that you can create a single app and load it with different features like SMS, Calls, Fax etc or create separate apps to separate these feature via separate API calls distinguished by different Client Id.
The main purpose of creating different apps is to separate these features and call it separately as I mentioned and also other reason is, an organisation can have different clients with different requirements. So based on that we can create different apps and provide it to different clients based on their need. So, each application given to each client will have different Client Id which mark the identity of the app.
RingCentral API already have API references describing each API in the API reference page where you will get all details and input parameters for each APIs.
There was a postman collection as well here in the reference forum answers but not sure if it is still there and still valid.
