Here's a quick attempt:
- app -- how a link/image/video is opened, and how you see what can be seen and how you can do what can be done. It could do filtering, but then you might waste resources on the platform downloading things you won't see.
- the platform -- determines all actions you can take that can be seen by other users, what settings admins can use
- the instance -- can toggle the aforementioned settings, write scripts for automated actions (even things not allowed by the API), change the web app through styles