You select on the remote which devices you want for IR and which for RF. RF sends to a hub unit that has IR blasters attached that you can route to your devices. The handheld remote sends IR signals to devices you haven't selected for RF.
Wait, so is it possible to remote a cable box that uses RF and not IR with the Harmony 900?
I don't know that the remote can interface directly to the cable box. I seem to remember something about it in this thread. Perhaps if you searched this thread with the model of your cable box you can find some info.