Let's follow up this discussion at the Neptune Community
(this forum will be kept in read-only mode)
Were you wondering what those methods in the IF_NAD_SERVER interface are for ?
We added some more info:
In SAPUI5 apps you will mostly use: HANDLE_ON_AJAX, HANDLE_ON_SYNC_OUT (if you use sap.n database sync framework), and maybe HANDLE_ON_RESPONSE if you ever need to reply to the browser with specific HTML/File content.