I think I understand... please correct me if this is wrong.

Since Infra red X-10 commands from a Pronto or and ALLforONE remote set for X-10 are NOT house code specific, they will trigger all IR receivers (IR543) in their line of site. And even though those receivers might be set for different house codes, the X-10 commands they emit will collide and interfere, or so I've been told.

I've got two RF transceivers, though, and noticed that all the RF emitters I've collected (slimpad, IBM Home Controller, palmpad, etc.) have their own house code to set, and so only send codes that a transceiver set to the same code will receive. A quick experiment confirmed that one transceiver set to house code "B" ignores any commands from a slimpad set to house code "A". The transceiver set to "A" receives these commands and successfully passes them thru to the devices they control.

So, using RF and multiple RF transceivers I can use device commands on more that one house code.
Using IR and multiple IR543s set to different house codes will only work if the IR543s are in different rooms and NOT activated at the same time.

You experienced X-10ers surely know this already. I found it to be VERY confusing, and not well explained in any of the documentation received with my X-10 devices. I hope this helps any newbies.

Dave