We had some discussions around the roles for such libraries, below are some guidelines:
Belt would be the most comprehensive library, if you can afford Belt, Belt is the only library you should rely on. There are some missing features in Belt, we plan to add it later, contributions are welcome.
JS is planned for migration from Js to ReScript, so it is mostly external bindings, it is in theory less intrusive than Belt when introducing ReScript into your existing codebase, contributions to more JS APIs are welcome.
Pervasives/OCaml stdlib. This is the library we inherited from OCaml, it will be kept as is. We may put it under a separate namespace called Caml.