Flash,
Even though most devices don't have seperate buttons for power on and power off on the remote, many of those devices were designed to accept such signals - even if they don't provide a means to send them with the supplied remote! They are programmed into the unit for use by custom installers and other folks.

One of the huge virtues of the MX900 and other higher end remotes is that they maintain a database of such commands. Not all gear has them, but many many do. In fact, I won't buy a component that doesn't support such discrete power commands - although I couldn't care less whether they are available from the supplied remote or not.

And yes, the frequencies are different for the toggle power on/off and the discrete power on and power off.