The universal server may or may not return anything. It just executes whatever is passed to it. In Erlang there are two ways of "calling" (in quotes for a reason). There's conventional function calling which is the regular synchronous style that we all know and love:

  f(10).

This will produce a result and return it to its caller. The other isn't really a call, it's "sending":

  Pid ! 10.

Some process id has been sent the value 10. It may be on this same node, it may be on another node, I don't have to care (sometimes I do though). This is asynchronous. Once a send is done the sending process will continue on (perhaps even terminating). At the other end of the send is a receive (hopefully, otherwise somebody's queue is getting filled up...):

  receive
    N -> ... % do something with this value
  end.

In the case of `universal_server` we don't know what it will become, it's just going to execute whatever 0-ary function is passed. That function may or may not include a "return" (sending a value back to the origin). It could also just terminate the universal server. Or it could temporarily convert the universal server into something else and then become a universal server again.

Slightly different in the low-level details, but conceptually accurate.