Both, but for the backend it's an entire framework that replaces something like Express/NestJS and for the frontend it is not. In the frontend can be used the validator and serialiser, so that types can be easily shared between frontend and backend. There is also a frontend desktop UI kit, but that is not the main focus (and was mainly separeted in a package because it was developed for GUI apps like API Console and Framework Debugger in Deepkit). You should use in the vast majority of cases the regular React/Angular/Vue frontend stack.
There’s a library called @deepkit/desktop-ui and another @deepkit/mongo. I guess you can use it anywhere, just pick libraries that makes sense on your environnement.