Me and my collegues like to talk about the "back-end of the frontend". This includes all the client-side logic and scripting. You have a server-side application primarily consisting of validation and data storage concerns, and a back-end of the front-end implementing some business rules etc.
This is fun, in my last job I used to call "back-end of the frontend" the nodejs server (of which, as a FE developer, I was in charge too) that talked to the "real" backend and aggregated data from various other sources. So in your schema that would have been "the backend of the frontend's backend" :)
lol.. I really like having a Node server for the app to talk to... for the most part it removes a lot of barriers to basic data transformations for requests and merging results closer to other data sources.
Not having to think in a different language helps a lot too.
