From the post: > Even without explicit mode annotations, the compiler can statically determine which variables may escape their enclosing region. Such variable

From the post: > Even without explicit mode annotations, the compiler can statically determine which variables may escape their enclosing region. Such variables are assigned the global mode; all others are automatically inferred to be local. There's a little more info about mode inference in the proposal: https://github.com/ocaml-flambda/ocaml-jst/blob/main/jane/doc/local-intro.md#inference https://github.com/ocaml-flambda/ocaml-jst/blob/main/jane/do... So arguments to "public functions" do require explicit annotations in order to be local, but otherwise the compiler is able to infer locality and this acts as a transparent optimization.