Some procedures or expression forms have no need for a result value. For example, the display procedure is called only for the side-effect of writing output. In such cases the result value is normally a special constant that prints as #<void>. When the result of an expression is simply #<void>, the REPL does not print anything.
A constant that prints as #<undefined> is used as the result of a reference to a local binding when the binding is not yet initialized. Such early references are not possible for bindings that correspond to procedure arguments, let bindings, or let* bindings; early reference requires a recursive binding context, such as letrec or local defines in a procedure body. Also, early references to top-level and module-level bindings raise an exception, instead of producing #<undefined>. For these reasons, #<undefined> rarely appears.