We use the /ust switch on the command line to give each client an I.D.
You can then send messages back from the urouter to the client if you have passed the $instancepath information down to the urouter.

Instancepath is generated for you. It is unique to the particular session of uniface running, we, however, run a windows batch file before uniface which forms the ust for the session from the logged in user name and a random number. (See below), this means that the Ust, which forms part of the instancepath is unique (and allows the user to run more than one uniface session from the same client).

start "" "C:\Program Files\Compuware\UNIFACE9\bin\uniface.exe" /asn=%1 /ust="%username:~0,8%%random%" demo tcp:

This makes my instancepath (for my current session). "TCP:+13001|i.sharp||i.sharp8549", so the UST forms part of the instancepath, and making that unique identifies your session. You can route messages from the urouter even more if you include :$instancename (So "%%$instancepath%%%:%%$instancename%%%"). Note that both of these are to handle explicit messages passed back from our services using postmessage (and we have to pass the 'reply address' in as a parameter, you can't (to my knowledge) find it out from inside the userver.

In the case of OUT parameters from the services, I think it's handled natively, but since we've had this setup for 10 years otr so now, I forget the teething troubles….